NSW Health has issued the warning, saying domestic drinking water tanks "could have become contaminated from debris, ash, dead animals or fire retardants".

There is also a chance the same debris flowing into Blowering Dam could hurt the fish populations, although an urban water management expert says the likelihood of such an occurrence depended on just how much rain fell in the catchment area.

"That's one reservoir that if you have a large rainfall event in that catchment, the inflows are going to be of lower quality that they normally are," Stuart Khan from the University of NSW, said.

But the good news, Professor Khan said, is that because Blowering's catchment is so large, it would take substantial falls to cause enough run-off of sediment and debris to trigger a mass fish kill.

Depending on how much nitrogen flowed into Blowering's water, there was also the possibility of an outbreak of blue-green algae, he said.

Professor Khan said erosion in areas that had been burned out was also a risk around the Blowering foreshore.

NSW Health has issued some tips to residents in bushfire areas about how to deal with possible water tank contamination:

Detecting contamination

If the water tastes, looks or smells unusual, do not drink it or give it to animals. Water from a river or creek should never be used for drinking or preparing food unless it has been properly treated. Water drawn from deep bores or wells should be safe to use.

Boil water for drinking purposes

Bring water to a rolling boil by heating water until a continuous and rapid stream of air bubble is produced from the bottom of a pan or kettle. Kettles with automatic shut off switches are suitable.

Contamination

To guarantee no contamination you would be required to drain and clean the tank and allow it to refill with clean rainwater or fill with water purchased from a water carter.. If you cannot drain the tank, find alternative uses for the water other than food prep such as garden watering, fire-fighting, clothes washing, flushing toilets, or disinfect the water with chlorine. Remove the animal remains (empty the tank if you can) and disinfect tank water with chlorine.

How to disinfect your tank water using chlorine:

For every 1,000 litres of water in your tank you can safely add: 125 mL or 125 g of 4 per cent chlorine household bleach (avoid bleaches that contain detergents or perfumes), or 40 mL or 40 g of 12.5 per cent chlorine liquid swimming pool or dairy factory chlorine, or 8 mL or 8 g of 65 per cent granular 'swimming pool' chlorine.Note: one teaspoon is about five grams. After chlorinating, you should wait at least 24 hours before using the water to allow for microorganisms to be destroyed

Fire retardants

If you believe your water source is contaminated by fire retardants, the requirements for disinfection are more stringent. You should not seek to disinfect the tank and should dispose of the contents. Before disinfecting and refilling from an authorised water carter. Boiling water does not remove fire retardants.