While fire-affected communities across Australia are praying for rain, there are concerns about what will happen when ash from the fire grounds is washed into waterways.

Key points: Tenterfield residents are boiling their drinking water after bushfire ash contaminated the dam

Tenterfield residents are boiling their drinking water after bushfire ash contaminated the dam A hydrologist says as the risk of major bushfires increases, governments will need to invest in infrastructure to keep water safe

A hydrologist says as the risk of major bushfires increases, governments will need to invest in infrastructure to keep water safe Bacteria can deoxygenate water by consuming organic matter such as bushfire ash, causing blackwater events that kill aquatic life

A forest hydrologist fears water contamination will be a major problem when burnt-out areas receive heavy rainfall.

Professor Patrick Lane from Melbourne University's School of Ecosystem and Forest Sciences said steep, dry-forested areas in particular were prone to mud slides after bushfires.

"They can shift enormous amounts of material into a stream network or a dam really quickly, and in that case the water supply can be contaminated for months on end," Professor Lane said.

There has already been an example of the potential problem in northern New South Wales.

A recent downpour of 36 millimetres increased the level of the Tenterfield Dam by 1.3 metres.

The Tenterfield Shire Council's general manager, Terry Dodds, said while the rain was welcome in a drought-affected community, it also caused some problems.

"There was very little vegetation in the burnt-out area to slow the runoff down," he said.

"The run-off that we got was pretty significant and very fast.

"The turbidity has now gone up 10 times higher than it was."

Boil tap water before drinking

Bushfire ash contaminated the runoff after much-need rain in Tenterfield. ( Image supplied: Julie King )

Air-borne ash from the fires that have been burning in the area for the past two months had already contaminated the dam, forcing locals to boil their tap water before drinking.

Mr Dodds said the problem was now far more severe.

"So the boil water alert, thank goodness we'd left it on — it was teetering to the point where we may have been able to lift it — but now it's 10 times worse than it's ever been," he said.

Residents have been contacting the Tenterfield Shire Council to complain about the water's taste and smell since the downpour.

Mayor Peter Petty said a reverse-osmosis water treatment system was being installed and should be operating within two weeks.

"It's got a smoke odour about it obviously, we can't take the odour out of the water," he said.

"The boil alert is still there and if you boil the water well it's fine … the water gets tested once or twice and day and that's seven days a week."

Water contamination on a massive scale

Ash blankets the forest floor in the Tenterfield Shire. ( Image supplied: Stephen Axford )

But with more than two million hectares of land already scorched across New South Wales alone this fire season, water authorities might have to confront similar problems on a massive scale.

Professor Patrick Lane said governments might need to invest in water infrastructure to cope with a potential increase in the amount and frequency of ash-contaminated catchments.

"I think water authorities in the past probably thought this was something that would come along once every now and again, but I think we need to be thinking much more about this could be something that happens every few years," he said.

"Putting the infrastructure in to deal with that economically is a big number, but delivering water to people, of course, is an absolute priority."

Ash clogs fishes gills

Ash contamination could also have a potentially devastating impact on aquatic ecosystems.

Craig Copeland is the chief executive and founder of OzFish Unlimited, and has been studying the protection and restoration of fish habitat throughout Australia for 30 years.

He said after major bushfires in Victoria and around Canberra, large fish kills were almost inevitable.

"It's a bad situation," he said. A dead fish found floating in ash contaminated water at Tenterfield. ( Image supplied: Brett Dowd. )

"The ash is going to wash into the water, it's going to cover all the food sources or a lot of the food sources for the fish, and it's going to clog up the gills of the fish.

"There's also going to be a set of bacteria in the water that are going to chew up the ash, and particularly in small water bodies they're going to chew up all the oxygen in the water.

"All we can hope for is that it is a large amount of rainfall and it just keeps washing through."