Mass fish deaths in the Murray Darling were not caused by drought, rather mismanagement of the river by state and federal governments and water agencies, new research says.
Up to a million fish died in the Darling River at Menindee early last week when a cool change swept through the region after a hot period.
Authorities believe the change may have killed off an existing algal bloom and depleted oxygen which worsened water quality for the fish.
The NSW and federal governments have been under pressure by scientists who say mismanagement of water is to blame.
On Saturday, a paper released by the Australia Institute said while drought was the catalyst for the critical ecological incident, drainage of the lakes in 2016 and 2017 of almost the equivalent of two Sydney Harbours was "not a natural phenomenon but a management decision".
While similar drainage has occurred previously, inflows to refill the lakes from the Northern Basin have also recently declined, the paper says.
"It is clear what has caused the Darling River fish kill - mismanagement and repeated policy failure," Australia Institute senior researcher Maryanne Slattery said in a statement.
"To blame the fish kill on the drought is a cop-out, it is because water releases were made from the Lakes when this simply shouldn't have happened," Ms Slattery said.
The report, A fish kill QandA, says the lakes were drained to save evaporation and to possibly justify the Menindee Lakes Water Saving and other projects.
But NSW Regional Water Minister Niall Blair dismissed the latter claim. He said there had been many releases over the years and many people were responsible.
"Hindsight is fantastic," Mr Blair told AAP on Saturday afternoon.
When the water was ordered and released, modelling showed it would be replaced by inflows, he said. Decision-makers would have factored-in this information.
"We are now debating whether they were right or wrong...this is the lowest inflows we've ever seen," he said, adding the drought had affected the situation.
The report says the Murray-Darling Basin Authority is partly responsible as it co-ordinated the management of the lakes for most of the time during the releases.
In response, the MDBA released a statement saying is was "confident the Menindee Lakes have been operated using the best information available" and the report was a "narrow view of operational issues".
When the Lakes rise above 640 GL, the MDMA can direct water to be released to meet downstream demands, but the NSW government takes control when storage drops below 480 GL, the MDBA said.
The report says claims by the NSW government that the incident was "under the control of Canberra" are false as releases by the MDMA cannot be made without approval by the Basin Officials Committee, which is directed by the state governments.
Recommendations of the report include making public annual reviews and all decisions of the Basin Officials Committee public.
Australian Associated Press