Mass fish deaths in the lower Darling River system were "preceded and affected by exceptional climatic conditions, unparalleled in the observed climate record", a new report says.
The federal government on Thursday released the interim findings of an independent review of the fish deaths.
Federal Agriculture and Water Minister David Littleproud said work would begin immediately to improve fish movement, create an early warning system for future events and do further research.
The report by Professor Rob Vertessy found flows on the Darling River at Bourke and Wilcannia were the lowest in 20 years.
The hot conditions resulted in significant algal blooms, which stayed still because of the low flow.
"High fish numbers and algal biomass became concentrated in the (surface water) and hypoxic or anoxic conditions developed in the (bottom waters)," the report said.
"Sudden reductions in air temperature and increased wind associated with storms caused water in the weir pools to suddenly de-stratify, resulting in low oxygen water throughout the water column and no escape for the fish. This was the primary cause of the fish deaths."
The report called for an assessment to identify parts of the Murray-Darling Basin most at risk of fish death events, to help with an early warning and crisis intervention strategy.
Gaps in water quality monitoring should be addressed, and barriers to fish movement removed.
In the short-term, efforts to prevent further fish deaths, such as the use of aerators and relocating fish should continue, noting that they may not prevent additional fish death events if there are further adverse conditions.
"I welcome the findings of the independent panel and will begin work in response immediately, including on allowing fish to move more freely around the river system," Mr Littleproud said.
Australian Associated Press