The lack of environmental water in the northern Murray-Darling Basin was the "root cause" of the massive fish kills near Menindee, and the "viability of the Darling" is at risk without urgent action, a study by the Australian Academy of Science has found.

The study, commissioned by federal Labor leader Bill Shorten, also calls for the repeal of the Northern Basin Amendment last year that cut the planned environmental savings for the region by 70 billion litres a year. It also backs the scrapping of a cap on water buybacks, and the suspension of a plan to re-engineer the Menindee Lakes.

The third and possibly largest of three big fish kills on the Darling River at Menindee, in a photo taken on January 29. Credit:Graeme McCrabb

Craig Moritz, chairman of the multi-disciplinary team that compiled the report, said the sight of millions of dead fish on the lower Darling River should serve as "a wake-up call", akin to a "coral bleaching event for the mainland".

While the region remains in the grip of a severe drought, conditions on the river were worse than could be attributed to the lack of rainfall because authorities had failed to take appropriate steps to prepare, said Professor Moritz, a biologist at the Australian National University.