Residents of Menindee are awaiting results of water quality testing after a mass fish kill event has locals concerned their tap water could be unsafe to drink.

Menindee Local Aboriginal Land Council director Michelle Kelly said the township was carting in fresh drinking water, as fears of waterborne disease remained high in the community.

"We have got people living on the river with babies [who] can't flush their toilets, can't have a shower or can't use the water and that's a real health issue out here," Ms Kelly said.

Millions of native fish were found floating down the Darling River system near Menindee in far west New South Wales on Friday morning.

The fish — predominately bony bream, but also golden perch, silver perch, and some Murray cod — died from low oxygen levels in the river brought on from high amounts of organic matter being washed into the system as recent flood waters receded, combined with high temperatures.

Local authorities have confirmed the overall scale and size of the mass fish death has eclipsed similar events in the area in 2018 and 2019.

Water utility Essential Water said independent tests on the water in the past week had detected no abnormalities to make it unsafe to drink.

But an Environment Protection Authority representative said extra water samples were flown to Sydney on Tuesday evening to be tested for heavy metals and pesticides.

On Sunday, State Emergency Operations Controller, Deputy Commissioner Emergency Management Peter Thurtell said in statement: "There is no need for community concern as the initial assessment has determined multiple viable solutions to maintain water supply to the Menindee township and surrounds".

"Importantly, there is ongoing testing of the water quality, which will ensure immediate action if a switchover for supply is required," he said.

Concerns flow downstream

The fish deaths have prompted South Australian authorities to more closely monitor local waterways, which are also susceptible to similar stresses if placed under high temperatures for prolonged periods.

Invasive Species Unit general manager from the Department for Primary Industries SA (PIRSA) Brad Page said while the conditions leading to the major fish kill seen in Menindee were different to what was experienced in other parts of the river system, the Murray could also be impacted.

"We had a big flood that’s gone out onto the flood plains, [the fish] have regrouped … and the trees washed back into the Darling river, which is like compost going into the river," he said.

"When we get really warm conditions … we see the boom in bacterial growth and microbes in the water, and essentially all of the oxygen’s been stripped by these microbes … and the fish suffocate.

SA Minister for primary industries and regional development Clare Scriven said there is nothing currently to suggest the River Murray will see fish kills on a mass scale.

Ms Scriven said current River Murray flows across the SA border reduce the risk of a similar event occurring in her state.

Clean-up begins

Contractors arrived in Menindee early on Wednesday morning with fish nets to begin the mammoth task of scooping out the amassed fish carcasses from the Darling River, and disposing of the remains.

New South Wales DPI Fisheries director of freshwater environment Cameron Lay said the carcasses that had pooled around the township would be targeted first.

"Obviously there is a huge number of fish that have perished over the last three to four days," he said.

"The smell and the visual impact is really not a pleasant sight for the local community."

Calls for investigation into river management

As clean-up begins upstream Ms Kelly said an independent investigation into the management of the Menindee Lakes and Darling River system was needed.