Multi-barrier approach

A multi-barrier treatment approach is a key principle of safe drinking water.

Multi-barrier approach: A key lesson from the Havelock North outbreak

In 2016, the Havelock North drinking water supply was contaminated by campylobacter bacteria. This had a devastating impact on the community. It caused more than 8,000 people to become sick from campylobacteriosis and contributed to four deaths.

A key lesson was that drinking water suppliers must maintain a range of safeguards to prevent people getting sick from their drinking water. In the water sector, we call this taking a ‘multi-barrier approach’ to drinking water safety.

What is a multi-barrier approach?

Drinking water comes from a range of places, including lakes, rivers, streams, and groundwater. But if you drink water straight from these sources there’s a risk that it could be contaminated with bacteria, viruses, or other things and make you sick.

So, drinking water suppliers must use a range of processes, procedures and tools to protect and treat water, to make it safe and keep it safe as it travels to the people who drink it.

This approach means that there’s always a range of safeguards (or multiple barriers) in place to make sure hazards and risks that might affect drinking water are addressed appropriately and the water stays safe.

This is important for two reasons.

First, no single barrier is effective against all types of contamination or risks.

For example, chlorine, which is commonly used to kill bacteria and viruses in drinking water doesn’t protect against protozoa.

Protozoa are single-celled parasites, like cryptosporidium and giardia, that can make people very . But they can be removed from water with filters or inactivated using ultraviolet (UV) light so they don’t pose a health risk.

Second, the unexpected can happen.

While drinking water suppliers are responsible for managing any risks to their supplies, things can go wrong.

Having multiple barriers (safeguards) in place means that if one fails, others are there as back-ups to help prevent, or reduce, the chance of public health being impacted.

Four key areas to consider as part of a multi-barrier approach

Drinking water suppliers must consider the following four areas when they develop their multi-barrier treatment approach to ensure they provide their community with safe drinking water.

Protecting the source water

The water that comes from your tap starts as rain, which runs over the land, and into rivers and under the ground, where it becomes groundwater. The water sector calls this ‘source water’.

It’s easier to treat water that has fewer contaminants in it. So, it’s important to prevent lakes, rivers, streams, and groundwater from becoming contaminated.

At a minimum, drinking water suppliers must know about the hazards and risks to their source water, and have a plan for how they will deal with them. However, taking steps to improve source water can make other aspects of running a drinking water supply much more effective and efficient.

Removing particles from the water during treatment

Removing dirt, leaves, and other particles is usually the first step that drinking water suppliers take to make water safe to drink. These particles can be removed using:

  • screens, which remove larger particles when water is first collected from rivers, lakes and other sources
  • processes that make small particles stick together so that they’re easier to filter out of the water
  • filters or membranes, which remove smaller particles and some bacteria, viruses or other micro-organisms.

Did you know?

There are lots of different water filters and filtration systems. Some only remove large particles, while others can remove almost everything from the water.

Disinfecting the water during treatment

Next, the water is disinfected to remove micro-organisms in the water that could make you sick.

When people say things like ‘germs’ and ‘bugs’, they’re often talking about micro-organisms. These are living things so small they can only be seen using a microscope. Viruses, bacteria, and protozoa are common types of micro-organisms.

When a drinking water supplier disinfects water in the treatment plant, this is called ‘primary disinfection’. At this stage, water can be disinfected by:

  • killing micro-organisms, usually using chlorine
  • ‘inactivating’ micro-organisms by using ultraviolet (UV) light to make them harmless
  • removing micro-organisms, using very specialised filters.

Protecting treated water

Once drinking water suppliers have worked hard to treat water to make it safe for their community to drink, they want to keep it safe.

A range of barriers (safeguards) can be used to do this, for example:

  • ensuring pipes that carry treated water to homes are kept in good repair
  • keeping treated drinking water reservoirs in good repair
  • ensuring any work done on pipes and reservoirs is done in a hygienic way
  • controlling pests around treatment plants and drinking water reservoirs
  • adding small amounts of chlorine to keep water disinfected while it travels through pipes to homes (in the water sector, this is called ‘residual disinfection’)
  • using equipment to prevent ‘backflow’.

What’s backflow?

Water networks are designed to send water from the plant to the people who use it. However, problems with pressure in the pipes can cause contaminated water, or other liquids, to be sucked into the water network from a tap, hose, trough, or other place where treated water usually flows out. If this happens, it’s called backflow.

The Swiss cheese safety model

This model was created in 1991 by James Reason, a professor at Manchester University. It’s widely used in healthcare, and other sectors, to help prevent accidents from happening.

Things can go wrong in complex systems. The Swiss cheese model uses the visual idea of Swiss cheese slices to help people see that having multiple lines of defence against risks is the best way to keep people safe from harm.

Here’s a diagram that shows how this model applies to the water sector:

The Swiss cheese model adapted for the water sector

Every slice of Swiss cheese has holes in it. The size and number of these holes are different on every slice.

Every barrier (or in this model, slice of Swiss cheese) put in place to protect against a risk could fail in different ways. For example, machinery could break down, no matter how well maintained it is, or a person could make a mistake. 

But another ‘slice of Swiss cheese’ is there providing back-up barriers to help protect against any one failure from harming people.

When it comes to safe drinking water, having multiple treatment barriers in place to minimise the risk of water making people sick was highlighted as essential during the inquiry that followed the 2016 Havelock waterborne illness outbreak.

Legislation and rules

For a multi-barrier treatment approach to be effective, drinking water suppliers must meet the requirements of Water Services Act 2021 and Drinking Water Quality Assurance Rules.

Water Services Act 2021

Many drinking water suppliers are required to have a Drinking Water Safety Plan. Among other things, these must show how suppliers will take a multi-barrier approach to drinking water safety.

Section 31(2) of the Water Services Act 2021 says:

A multi-barrier approach to drinking water safety is one that Taumata Arowai considers will—

(a) prevent hazards from entering the raw water; and

(b) remove particles, pathogens, and chemical and radiological hazards from the water; and

(c) kill or inactivate pathogens in the water; and

(d) maintain the quality of water in the reticulation system.

Go to the legislation

Drinking Water Quality Assurance Rules

Drinking Water Quality Assurance Rules (the Rules) set out the minimum treatment and monitoring requirements a drinking water supplier must meet.

The Rules align with international practices and are tailored for different and the size of the community served.

In addition, drinking water suppliers must have an up-to-date Drinking Water Safety Plan (DWSP), unless they’ve adopted an Acceptable Solution or have an exemption.

Among other things, DWSPs need to clearly explain how the supplier is implementing a multi-barrier approach to drinking water safety.

Acceptable Solutions give small drinking water supplies options to provide safe drinking water, without meeting all of the requirements above.

Find out more




Our role as regulator: multi-barrier approach

As New Zealand’s water services regulator, our aim is safe water, every day for everyone.

Achieving this aim is only possible if all drinking water suppliers have an effective multi-barrier treatment approach in place.

We make sure drinking water suppliers are taking a multi-barrier approach by:

To have an effective multi-barrier approach, a drinking water supplier must:

  • have effective barriers in place for all types of possible contamination
  • meet the minimum requirements in the Rules
  • have appropriate monitoring in place
  • have well-trained, experienced team members managing drinking water supplies who:
    • understand the minimum requirements set out in the Rules to ensure safe water is provided to the community
    • are aware of the kinds of hazards that can impact a drinking water supply
    • proactively identify and manage risks from any potential hazards through active use of an up-to-date Drinking Water Safety Plan and awareness of how their supply operates and performs.