A team of CSIRO scientists has warned more fish deaths and algal blooms in the River Murray could result from ash and debris washing into waterways near bushfire grounds.

Key points: Ash and loose soil could create ideal environment for algae blooms, CSIRO microbiologist says

Ash and loose soil could create ideal environment for algae blooms, CSIRO microbiologist says Government agencies closely monitoring threat of bushfire contamination in River Murray

Government agencies closely monitoring threat of bushfire contamination in River Murray MDBA says bushfire-related poor water quality unlikely to spread far past Hume Dam

More than 200,000 hectares have been scorched above the Hume Dam in New South Wales, which is the River Murray's main store of water supply.

"What we get after a major fire event like this is sediment, ash and debris washing off the catchments into the waterways," CSIRO microbiologist Gavin Rees said.

"This brings with it the potential to cause a fair bit of environmental damage or stress to the river systems."

Australia has experienced what's been described as an unprecedented bushfire season. ( Supplied: Gena Dray, file photo )

Dr Rees said the influx of ash and loose soil could lead to fish deaths — some on a large scale.

"It's [fish deaths] very likely where you've got a small body of water meandering its way through many kilometres of burnt-out forest," he said.

He also said ash and soil could lead to deoxygenation in parts of the River Murray, and create the ideal environment for algae.

"[Bushfire] material can stimulate the microbes usually present in the river and they can start sucking up a lot of the oxygen in the water column," he said.

"You can have large tracks of water where there's no oxygen."

Bushfire debris in Hume Dam

The Murray Darling Basin Authority said it was looking at how sediment from bushfires might move through the stream network in the Upper Murray catchment.

Its executive director of river operations, Andrew Reynolds, said the impact on water quality would be felt in streams upstream of the Hume Dam and potentially in the upper reaches of Lake Hume.

A mass fish kill happened in the Macleay River, on the mid-North coast, earlier this year. ( Supplied )

"We'll continue to monitor how the lake performs and whether those impacts are starting to move through the lake," he said.

"But I certainly wouldn't expect that poor quality resulting from the bushfires to extend far beyond Hume Dam itself."

But while large amounts of bushfire debris would be unlikely to travel past the Hume Dam, Dr Rees said the flow-on effects could be seen in western Victoria and South Australia.

"We could see a legacy effect where, during summers, blooms become a little more frequent," he said.

"That could happen in the Riverland in South Australia's end of the river."

The Murray River at Loch Luna in South Australia's Riverland. ( Supplied: Isaac Barber )

In 2016, an algal bloom that started in Lake Hume spread 700 kilometres down the river, triggering spots of algae halfway between Mildura and the South Australian border.

Algal blooms can make rivers unsafe for all users of water, including for agriculture, irrigation and recreation, and therefore dry up an area's tourism sector.

Water authorities on high alert

The Commonwealth Environmental Water Holder, Jody Swirepik, has echoed the CSIRO's warnings but does not believe South Australia will be heavily impacted.

"If you look at where the fire has passed through, the most significant part in the Basin is in Upper Murray catchment above the Hume Dam," Ms Swirepik said.

"For people in the lower catchment I wouldn't say there's a lot of calls for concern at the moment."

Rains are expected to wash ash and debris from bushfire sites into waterways. ( Screenshot )

The Environmental Water Holder said she was monitoring the threat of bushfire contamination closely and there was the capacity to use environmental water to mitigate damage.

"The way we could respond is to actually order water to come in a particular pattern that we think will mitigate the water quality problems," Ms Swirepik said.

"We could alter the flow pattern to be able to provide some relief to any water quality issues."

"In 2016, where there was flooding and that was bringing Blackwater down the river, we were able to add fresher water into the river."

But as for stopping the ash and debris washing into waterways in the first place, Mr Reynolds said it was near-impossible.

"Because the fires have been quite widespread and big areas have been covered, it's very difficult to prevent that contaminated material washing into streams … there's probably very limited on-ground work that we could do that would be effective," he said.