Discharge to water standard
The standard sets wastewater treatment requirements based on the sensitivity of the receiving environment.
The standard sets treatment requirements for the main contaminants released from wastewater treatment plants into waterbodies. The treatment requirements depend on how sensitive the receiving waterbody is and the level of environmental risk.
The standard has three steps to follow.
- Identify the type of receiving waterbody the wastewater is discharged to.
- Check the concentration limits for each contaminant and what monitoring and reporting is needed.
- Include and implement the requirements in the resource consent conditions.
Types of waterbodies covered by the standard include:
- lakes
- rivers and streams (with high, moderate, low, or very low dilution)
- estuaries
- low-energy coastal waters
- high-energy coastal waters
- open ocean.
Pollutants covered by the standard include:
- organic matter (measured as cBOD₅)
- Total Suspended Solids (TSS)
- Total nitrogen and phosphorus
- ammonia (ammoniacal nitrogen
- E. coli
- enterococci.
Small wastewater treatment plants have different discharge to water requirements. These plants differ significantly to those that service larger towns and cities.
Wastewater standards are implemented by consenting authorities as conditions of consent when new or renewed wastewater resource consents are granted.
Where a regional plan enables discharges of treated wastewater to water, a consenting authority cannot grant a resource consent that imposes requirements that are different from those in the standards. This includes treatment limits for core contaminants, as well as monitoring and reporting requirements. Where a consent is approved, it must be issued for a 35-year duration.
Consenting authorities maintain discretion over matters not covered by the standards, e.g. the volume and timing of treated wastewater being discharged and limits for other contaminants.
Find out more about timeframes for transitioning to the new standards.
Applicability and exceptions
Standards only apply to public networks, which are primarily owned and operated by local councils. They do not apply to:
- privately-owned wastewater treatment networks or septic tanks
- discharges from wastewater treatment plants that fall within the scope of the Australian/New Zealand Standard AS/NZS 1547:2012 (e.g. campground wastewater systems)
- where there are inconsistencies with aspects of legislated treaty settlement obligations in the Waikato-Waipā, Whanganui and Whangaehu river catchments.
The discharge to water standard does not apply to certain water bodies, pristine environments, drinking water abstraction points, bypasses or small plants, as detailed below.
Certain water bodies
Includes discharges to any aquifer, cave/karst system, or geothermal water. Due to high vulnerability, discharges to these systems are exempt from standards and any proposed discharge to these environments needs to be considered and managed by the consent authority.
Karst aquifers are defined as underground water systems where caves or conduits significantly contribute to the flow path.
An aquifer system is defined as a geological formation that can transmit and store groundwater. It is characterized by its hydraulic conductivity and transmissibility, which influence the movement of fluids and contaminants within the groundwater flow system.
Geothermal waterway is defined as water that originates from geothermal sources, often containing dissolved minerals and gases.
Sensitive (pristine) environments
Discharges to a water body that meets either of the following:
- meets all ‘A band’ attribute states specified in Appendix 2A of the National Policy Statement for Freshwater Management 2020outbound
- meets all ‘A band’ attribute states except any that the water body fails to meet due to naturally occurring processes.
If a waterbody meets the values of all Attribute Band A for all attributes, it will be considered a pristine freshwater body and is an exception to the standard.
A review of NPS-FM 2020 is currently being undertaken. Should this result in change or removal of the A Band attribute values, reference should be made to their values as of 19 December 2025.
Drinking water abstraction points
Discharges where the point of discharge is within:
- 1,000m upstream, or 100m downstream, of any drinking water abstraction point in a river
- 500m of any drinking water abstraction point in a lake
- 1,000m upstream of any tributary that flows into any lake, if the discharge point in the lake is located within 500m of a drinking water abstraction point.
The treatment limits in the standard are designed to meet contact recreation guidelines, but they do not meet drinking water quality standards. Any discharges proposed within the above proximities are an exception to the standard and must be considered and managed by the relevant consent authority.
Bypasses
The discharge to water standard does not apply to any discharge from, or associated with, any bypass of a wastewater treatment plant. Bypasses generally occur in times of significant rainfall, mechanical or plant failure, or when certain treatment processes are shut for maintenance or upgrade. During these times, a bypass is unlikely to achieve the limits required by the wastewater standards and is subsequently an exception to the standard. The monitoring and reporting requirements for bypasses are covered in the overflows and bypasses standard.
Small plant standard
A small wastewater treatment plant standard has been developed for existing small plants, recognising and providing for the unique settings and challenges that many of these plants have.
Receiving waterbody categories
To apply the standard to a discharge to water consent, the first step is to identify the type of receiving waterbody. To help streamline the consenting process, network operators and consenting authorities should discuss and agree on the waterbody type early on.
There are nine types of receiving waterbodies.
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Receiving waterbody |
Definition of category |
User notes |
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Freshwater receiving waterbody |
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Lake |
Body of standing freshwater, which is entirely or nearly surrounded by land. |
Includes lakes and natural ponds but excludes any artificial ponds. Typically, low-energy environment in which dispersion / dilution is limited by an absence of strong water currents. |
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Very low dilution river
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The dilution ratio at the point of discharge is 10 or less. |
Rivers are a continually flowing body of fresh water, including streams and modified watercourses. Excludes any artificial watercourse (including an irrigation canal, water supply race, canal for the supply of water for electricity power generation, and farm drainage canal). The definition also excludes a part of a river that is in the coastal marine area and any part of a river that is an estuary. River environments are categorised based on the dilution ratio of treated wastewater to volume of river flow at the point of discharge. |
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Low dilution river
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The dilution ratio at the point of discharge is more than 10 but 50 or less. |
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Medium dilution river |
The dilution ratio at the point of discharge is more than 50 but 250 or less. |
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High dilution river |
The dilution ratio at the point of discharge is more than 250. |
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Coastal receiving waterbody |
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Estuaries |
A body of water listed in the table in Schedule 3 of the standards. If the listed waterbody is a river, it only includes the part of the river that is within the coastal marine area. |
A partially enclosed coastal body of water that is either permanently or periodically open to the sea in which the aquatic ecosystem is affected by the physical and chemical characteristics of both runoff from the land and inflow from the sea. In determining if the estuary category applies, network operators and consenting authorities can consider tools such as the estuaries spatial database at https://www.doc.govt.nz/nature/habitats/estuaries/estuaries-spatial-database/outbound |
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Open ocean |
The point of discharge is –
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The criteria for classifying a receiving waterbody as open ocean are designed to enable an objective assessment of whether a proposed discharge meets this category. The 10m depth contour is both easily definable on the NZ Bathymetric Charts and typically occurs outside of the surf zone. |
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High-energy coastal |
The point of discharge is –
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This category applies to discharges into waters that are less than 500m from shore and shallower than 10m deep. It also considers the stronger mixing that can happen along open coastlines, similar to the open ocean category. The definition of ‘high-energy coastal’ can vary, so it is important to get expert advice early in the consent process. The applicant and the consent authority should agree on how the criteria apply. |
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Low-energy coastal |
The point of discharge is in the coastal marine area but is not an estuary, the open ocean or high-energy coastal water |
This category covers the rest of the coastal marine waters that are not included in the other categories. These areas are usually more sheltered from big, long waves. They are often found in places like gulfs, behind islands or reefs, and in sheltered harbours or bays. To decide whether an area fits into this low-energy coastal category or the nearshore category, it’s important to get expert advice early in the consent process. The applicant and the consent authority should agree on this together. |
Dilution and mixing requirements
Dilution ratio for freshwater
We use a dilution ratio because it’s simple, widely understood by regulators and practitioners, and avoids the need for complex modelling. Other countries like Canada, US, Switzerland and those in the EU also use this approach. It acts as a stand-in for how well the receiving waterbody can mix and absorb the discharge and helps show how big the discharge is compared to the size and flow of the waterbody.
To classify a river, the dilution ratio of wastewater to freshwater at the discharge point is calculated using the following formula.
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Dilution ratio = |
Qeffluent + Qannual low flow |
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Qeffluent |
Dilution and mixing for lakes and coastal waterbodies
The standard requires certain dilution outfall and mixing requirements to be achieved by discharges to lakes and coastal waterbodies as detailed below.
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Receiving waterbody |
Outfall and mixing requirements |
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Lakes |
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Estuaries |
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Low-energy coastal |
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High-energy coastal / nearshore |
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Open ocean |
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In granting consent for discharges into any of these waterbodies, applicants and consenting authorities must ensure that the design and mixing requirements are achieved by the proposed discharge.
If an existing outfall does not meet the required minimum centreline dilution standards, there are several options to improve initial dilution. These include installing check valves on the ports to increase plume velocity and enhance mixing or adding more ports to distribute the discharge more effectively. Alternatively, the outfall could be extended or relocated to an area with better dilution characteristics or one that qualifies as a different receiving waterbody type.
Modelling to understand dilution and mixing requirements
Centreline dilution modelling requirements include a thorough collection of data on the receiving waterbody (currents, tides, density, salinity), the profile of the discharge (flow rate, density, temperature) and the outfall geometry.
Key requirements are to define the mixing zone boundaries, use an appropriate numerical model like CORMIXoutbound to simulate dilution at the centreline, and then analyse the results to determine dilutions. There are three key steps in modelling dispersion and mixing as outlined below.
Understanding the receiving waterbody
- Detailed information on water current speeds, directions and tidal cycles is essential for accurately predicting the dilution of the discharge.
- Ambient water density and salinity are crucial for modelling the behaviour of a discharge.
- Understanding the vertical distribution of density is also required.
Understanding the profile of the discharge
- Density, temperature and flow rate of the effluent must be known.
- Details such as the outfall pipe's height, diameter and the orientation of the discharge are important for predicting the initial dilution (meeting the standard).
Modelling
- Use a recognised numerical model, such as CORMIX, which is designed to predict mixing zone behaviour. Input the collected data into the chosen model to simulate the effluent plume's trajectory and dilution.
Contaminants and concentration limits
When applying for a resource consent to discharge treated wastewater into a waterbody, applicants must include concentration limits for all contaminants covered by the standard. These contaminants were chosen because they represent the main environmental and health risks from treated wastewater discharges.
For the contaminants covered by this standard, regional councils responsible for wastewater regulation cannot impose stricter or more lenient treatment requirements.
Some contaminants, like per- and polyfluoroalkyl substances (PFAS) or heavy metals such as iron and aluminium, are not included in the standard. These may be less common or there is currently a lack sufficient evidence to set national limits. In such cases, consent authorities can continue to set treatment requirements as needed.
| cBOD₅ (Carbonaceous Biochemical Oxygen Demand) | Shows how much biodegradable material remains after treatment. High levels can reduce oxygen in water, affecting aquatic life. Monitored through dissolved oxygen levels. |
| TSS (Total Suspended Solids) |
Measures particles that reduce water clarity and can harm ecosystems. Effects are assessed through visual clarity and sediment build-up. |
| Ammoniacal Nitrogen (Amm-N) | Toxic to aquatic life in high amounts. Limits are based on Australian and New Zealand Guidelines for Freshwater and Marine Water Quality (ANZG, 2018)outbound toxicity guidelines for species protection. |
| Total Nitrogen (TN) |
Includes all nitrogen forms, contributing to nutrient pollution and algal growth. Limits follow ANZG (2018) nutrient guidelines. |
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Total Phosphorus (TP) |
Another nutrient that can drive algal blooms. Limits also follow ANZG (2018) guidelines. |
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Pathogen Indicators (e.g. E. coli, Enterococci) |
Used to assess health risks from recreation or shellfish consumption. |
View the concentration limits for the discharge to water standard.
Quantitative Microbial Risk Assessment (QMRA)
Shellfish gathering is a core part of New Zealand’s cultural identity. The discharge to water standard addresses this by requiring a Quantitative Microbial Risk Assessment (QMRA) in certain scenarios. QMRA is a scientific, evidence-based method used to assess potential public health risks associated with pathogens in treated wastewater discharges.
Under the standards, a QMRA is required when the point of discharge is:
- no more than four kilometres from a shellfish gathering area in a lake or the coastal marine area
- no more than four kilometres upstream from a shellfish gathering area in a river.
A QMRA must be undertaken by a Suitably Qualified and Experienced Practitioner (SQEP), independent of the applicant. The QMRA should demonstrate:
- advanced understanding of microbiology and pathogen behaviour
- experience with hydrodynamic and dilution modelling
- knowledge of dose-response relationships for key waterborne pathogens
- familiarity with New Zealand regulations and customary gathering practices.
Network operators and councils should agree early on whether a QMRA is needed to ensure the consent process is not delayed.
Periphyton risk assessment
Periphyton growth can disrupt ecosystems in hard-bottom rivers and streams, especially when nutrient levels are too high (Total Phosphorous and Total Nitrogen). To protect sensitive environments, discharges to these streams must undergo site-specific assessments to set appropriate nutrient limits and mitigation measures.
Under the standards, a periphyton risk assessment is required when, within 100m of a discharge point, more than half of the substrate (the material that makes up the bed of the river) is made up of particles that are the same size or larger than gravel.
Small wastewater treatment plants are exempt due to their low impact. However, if a plant grows beyond the “small” threshold and discharges to a hard-bottom stream, a periphyton risk assessment will be required.
A periphyton risk assessment must be undertaken by a Suitably Qualified and Experienced Practitioner (SQEP), independent of the applicant.
For guidance on identifying hard-bottom streams, assessment requirements, and interpreting results, see discharge to water guidance.
Monitoring and reporting requirements
Monitoring and reporting requirements are critical components of a resource consent to discharge treated wastewater. These requirements help build a reliable understanding of system performance, pollutant removal and compliance with consent conditions.
The standards establish a nationally consistent monitoring and reporting regime. Consent holders are responsible for transparently reporting all monitoring data and compliance outcomes. This enhances visibility of wastewater infrastructure performance for councils, communities and regulators, supporting environmental protection and public confidence.
Sampling
Sampling requirements under the standards vary based on the size of the population served by the wastewater treatment plant and whether the plant qualifies for the small plant standard.
Sampling frequency by plant size
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Plant size |
Sampling frequency |
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Large (>10,000 people) |
At least daily sampling of all required parameters |
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Medium (1,000–10,000 people) |
At least fortnightly sampling |
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Qualifying small plants |
At least quarterly sampling |
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Sampling must be carefully planned and carried out at the appropriate points in the treatment process. |
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Testing and analysis
Accurate testing is essential for understanding treatment performance and protecting receiving waterbodies.
Best practice for preserving the integrity of samples from field to laboratory is recommended. All analysis should be conducted by IANZ-accredited laboratories. However, alternative methods may be approved in writing by the consenting authority.
Reporting of results must include:
- compliant results that meet discharge limits
- non-compliant results that exceed thresholds
- routine monitoring data collected at regular intervals
- follow-up or intensified monitoring results undertaken after an exceedance or operational issue.
All results must be securely stored for at least 10 years in a form that allows for retrieval and verification.
Reporting requirements
A nationally consistent reporting regime ensures transparency and accountability. Reporting frequency depends on the size of the treatment plant.
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Plant size |
Reporting frequency |
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Large & medium (≥1,000 people) |
Monthly reporting |
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Qualifying small plants |
Quarterly reporting |
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All plants |
Annual reporting |
Reports must include:
- raw data and sampling results
- summary of compliance against concentration limits
- details of any exceedances, causes and corrective actions.
Reports must be:
- provided to the consenting authority
- sent to the Authority
- published on a publicly accessible website.
Elements not covered by standards
Resource consents for discharges to water from wastewater treatment plants must align with the standard. Consent conditions cannot contradict the standard unless specific exceptions apply.
While the standard sets clear requirements, consenting authorities retain discretion over discharge activity not covered by the standard. These may include:
- monitoring and treatment of other contaminants
- receiving waterbody monitoring, such as upstream/downstream sampling or cultural monitoring programmes
- timing and volume of discharges, provided they align with dilution ratio requirements
- community engagement, including liaison groups or consultation processes.
Consent authorities must ensure that any additional conditions do not inadvertently conflict with the standard. For example:
- a receiving waterbody limit for Total Nitrogen that requires more treatment than the standard’s end-of-pipe limit or requiring an alternative form of nitrogen to be treated
- an end-of-pipe concentration limit for contaminants like Per- and Polyfluoroalkyl Substances (PFAS) or heavy metals that demands treatment beyond what the standard requires.
Maintaining alignment with the standard ensures consistency, fairness and clarity in how wastewater discharges are managed across the country.
Small plant standard
A small wastewater treatment plant standard has been developed for existing small plants, recognising and providing for the unique settings and challenges that many of these plants have.
A wastewater treatment plant qualifies as a small wastewater plant if it meets either of these criteria:
- load-based qualification where the plant takes in an average total load of less than 85kg of cBOD5 in a day; or
- population-based qualification where the plant services fewer than the equivalent of 1,000 people (calculated as an annual average).
In either scenario, the intent to seek resource consent under the small plant standard should be discussed and agreed between applicant and consenting authority early in the consenting process.
Small vs large plants: key differences
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Normal standard requirements |
Small plant standard requirements |
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Concentration limits |
· Annual median limits for Total Nitrogen, Total Phosphorus, Total Suspended Solids, and cBOD₅. · 90th percentile limits for Total Suspended Solids, cBOD₅, Ammoniacal Nitrogen, and E. Coli or Enterococci. |
· No concentration limits for Total Nitrogen and Total Phosphorus (including where discharging to a hard-bottom stream). · Annual median limits for Total Suspended Solids and cBOD₅. · 75th percentile limits for Total Suspended Solids, cBOD₅, Ammoniacal Nitrogen and E. Coli or Enterococci. |
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Monitoring and reporting frequency |
· Either daily or fortnightly sampling required. · Monthly and annual reporting required. |
· Quarterly sampling required. · Quarterly and annual reporting required. |
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Operational requirements |
· No operational requirements imposed by the standards. |
· Requirement for regular sludge surveys and desludging triggers. |
Small plant status using load-based qualification
An influent cBOD₅ average load of less than 85kg per day has been developed to provide an objective and measurable qualifying criteria for small plants. This accounts for the variability in flow and load factors that are present when relying on population-based qualifying criteria.
In determining small plant status under this qualifier, the consenting authority will need to be satisfied that an adequate number and representative set of influent samples have been provided.
Samples need to be taken before primary treatment has occurred.
Ongoing influent cBOD₅ load monitoring should be required as a consent condition to ensure the small plant qualifying criteria are still met. This should occur at a frequency determined by the consenting authority.
Small plant status using the population-based qualification
As not all small plants monitor influent cBOD₅ loads, an alternative qualifying criteria has been developed. Plants serving a population equivalent of 1,000 people or fewer can qualify for the small plant standard. However, because of factors such as seasonal population fluctuations, abnormal connections, or the possibility for significant industrial or trade waste inputs, this qualifying pathway is subject to more variability.
A consent authority will need to consider the following when determining if a plant qualifies as a small plant under this pathway.
- The number of connections to the wastewater network.
- The estimated number of people served by each connection and any abnormal connections such as campgrounds or schools.
- Any seasonal variation in the population served by the network.
- Whether there is a significant proportion of trade waste or industrial inputs.
In determining small plant status under this pathway, the consent authority will need to be satisfied that estimates and assumptions regarding the above considerations are adequately provided for.
Verifying small plant status
Consent holders operating under the small plant standard must verify their status every 12 months by submitting qualifying information from the previous year to the consenting authority.
What happens next?
- The authority reviews the data to confirm continued eligibility.
- If the plant exceeds the 85kg daily influent cBOD₅ limit or if there are changes in population or inputs, the plant must:
- conduct monthly influent cBOD₅ monitoring for the next 12 months.
After 12 months
- If the average daily influent cBOD₅ is below 85kg, the plant may continue under the small plant standard.
- If it’s above 85kg, the plant must transition to the larger plant requirements.
This process provides flexibility for small plants experiencing fluctuations, while ensuring sustained exceedances are appropriately managed.
Transitioning from small plant status
If a plant continues to exceed the small plant qualifying criteria after 12 months of monitoring, it must transition to the requirements for larger plants. This includes:
- completing a Periphyton Risk Assessment (if discharging to a hard-bottom stream)
- meeting stricter treatment standards, including concentration limits for Total Nitrogen and Total Phosphorus, and shifting from 75th to 90th percentile limits
- complying with more frequent monitoring and reporting requirements.
Operators have three years to complete any necessary upgrades and achieve compliance. During this time, compliance can still be assessed against small plant requirements, allowing time to plan and implement changes.
Sludge management in oxidation ponds
Sludge naturally builds up at the bottom of oxidation ponds over time. Excess sludge can reduce treatment effectiveness by:
- decreasing pond volume and hydraulic retention time
- increasing odour risks and potential for sludge surfacing
- lowering the chance of converting sludge to biosolids.
Under the standard, small wastewater plants with oxidation ponds must:
- conduct a sludge survey every three years
- submit results to the consenting authority, including:
- sludge levels, volumes and proportions
- volatile solids content
- impact on treatment performance.
Based on survey results, desludging requirements will be set as consent conditions. These requirements remain in place even if the plant no longer qualifies as a small plant.
Consent conditions for small plants
To support long-term compliance, consenting authorities should enable the use of a single resource consent for small plants, with staged conditions that reflect the plant’s status over time (e.g. if a plant ceases to be small and new requirements apply).
Consent conditions should include:
- small plant requirements apply as long as the plant qualifies
- annual verification of small plant status
- 12 months of influent monitoring if the plant no longer meets qualifying criteria
- three-year transition period for upgrades or assessments, during which small plant conditions still apply
- application of large plant standards once upgrades are complete.
Conditions should clearly state when each stage applies. Where conditions become irrelevant, they should automatically cease—no formal change of conditions required.
This approach ensures clarity and flexibility throughout the life of a consent, including consents lasting up to 35 years.
An exception to the staged consent approach applies to small plants discharging to hard-bottom streams. In these cases, a periphyton risk assessment is required to determine appropriate concentration limits for Total Nitrogen and Total Phosphorus.
Because these limits depend on site-specific assessment, they cannot be set at the time of granting consent. As a result, a change of conditions will likely be needed later to impose the new limits.