Floods

High turbidity can stop treatment processes such as chlorination and UV light disinfection from working properly. A flood will usually raise the turbidity of your source water (especially streams and rivers), so it’s important to wait until turbidity returns to normal before re-establishing your supply.

You can test the turbidity of your source water using a hand-held device called a turbidimeter or send samples to an accredited laboratory for testing.

Check electrical systems (e.g. UV disinfection unit, pumps) carefully. If there’s any risk, wait until a qualified electrician can perform a full safety test.

Check the UV transmittance of your UV disinfection equipment to ensure it delivers the correct dose. Contact a specialist technician for assistance if required and continue to use an alternative water source until you’re sure the system is working properly.

Divert any water collected in storage tanks during the flood to waste, then clean and disinfect the tanks  – taking extreme care when working at height or within the confines of the tank. Make sure you wear personal protective equipment (PPE) and there’s someone nearby at all times in case you need help.

If you have plain unscented bleach available, you might be able to disinfect your tanks using the following process. Don’t use bleaches that have detergents/surfactants (i.e. foam up when shaken), fragrances (e.g. lemon-scented) or are in gel form. Ideally use liquid bleach which contains 5 - 6% sodium hypochlorite.

1. Calculate the volume of your tank
   Circular tank: 3.142 x radius (in metres) x radius (in metres) x depth of water (in metres) x 1000 = number of litres. Radius = half the diameter (the widest part of the circle).
   Square tank: Length (in metres) x width (in metres) x depth of water (in metres) x1000 = number of litres.
2. Add the required volume of unscented bleach shown in the table below and mix (you can use a broom handle or similar clean object to stir).

3. Turn on each tap and flush each toilet in the house or buildings until you smell chlorine and then close the tap. You are trying to get the chlorine solution to all parts of the plumbing. If there are any in-line filters, they should be removed and replaced with new filters after the disinfection is complete.

4. Allow the chlorine to sit in the pipes for at least two hours (preferably overnight).

5. Open all taps and flush out the chlorine solution for a couple of minutes to ensure it has passed through all pipes.

If your borehead or wellhead has been submerged, clear away any accumulated material and check whether it’s likely that silt or floodwater has entered the bore. Check for holes or gaps in the borehead structure.

Check also whether the flood is likely to have caused contaminants to enter the groundwater system at another point. Your local council might be able to help with this assessment.

If you think your bore might have been affected by the flooding, pump the bore to waste for 24 hours. If the bore is under water, do not pump.

Use a chlorine solution (1 teaspoon in a 10 litre bucket of water) to clean the bore seals, vent and pipework and then wash the concrete surround.

Download a factsheet on managing a groundwater supply after a flood 

Check your roof and storage tank thoroughly. Remove any obvious contamination sources such as leaves and pests and clean around the tank lid before opening.

If contaminated floodwater has entered the tank, you’ll need to empty, clean and disinfect it using chlorine (see above) before you use it again. Sediment can be removed using a swimming pool cleaner or by a contractor.

If the top of the tank is secure, the tank is undamaged, the roof is clean and there has been no contaminated water entering (a first flush diverter is in place and operational) then the water should be safe to drink. However, you might wish to arrange for an accredited laboratory to test the water for microbiological (germs) and chemical contaminants before advising your community that it’s safe to drink (see below).

If your roof appears to be contaminated (e.g. by windblown debris) disconnect the tank at the downpipe and wash the roof if it’s possible to do so safely. Brush the roof to dislodge any dust, silt or debris – remembering to wear a mask, goggles and personal protective equipment (PPE).

Download a factsheet on managing a roof water supply after a flood

If your treatment system has been offline and untreated or partially treated water has entered the supply, or you’re unsure whether your water is safe to use, arrange for an accredited laboratory to test your treated water for:

• microbiological contaminants (germs) - E. coli and total coliforms
• any chemicals and other contaminants (e.g. fuel) that you may be concerned about or that the laboratory recommends.

If laboratory tests show that your water contains chemicals that exceed their maximum acceptable valuable (MAV), keep the Do Not Use Water notice in place. Tell your community to continue using an alternative water source (e.g. bottled water, water from a registered water carrier).

If the water contains microbiological contaminants (E. coli or total coliforms), issue a Boil Water notice instead. Boil all water used for drinking, washing dishes by hand, brushing teeth, preparing food, mixing baby formula, making ice and giving to pets – until further notice. Note that boiling water won’t get rid of chemical contaminants.

If your supply is registered with us, notify us of the contamination as soon as possible. We can help you to re-establish a safe supply.

Don’t lift the notice until laboratory tests have given the all-clear.