Up to 26 gigalitres of water will be released into the Murrumbidgee in the NSW Riverina region with the river on high alert for further fish kills.
Algal blooms, high temperatures and low flows are affecting lower parts of the river - which is part of the Murray-Darling system - with up to 10,000 fish dying on the Murrumbidgee at Redbank Weir last week.
Over the weekend, Mathi-Mathi man Brendan Kennedy shot a video showing dozens of fish floating in stagnant water and on the riverbanks near the barrier.
“It just breaks my heart to see this,” he said.
“I couldn’t count all the dead fish here. It’s an absolute disgrace.”
The Murray-Darling Basin Authority on Monday said up to 26 gigalitres of environmental water will be released over the coming weeks into the river between Hay and Balranald to improve flows and prevent further fish deaths.
Environmental water is water set aside to protect or improve the environmental values of wetlands, rivers and other waterways.
"Environmental water holders are doing everything they can to help fish, other wildlife and vegetation survive the current extreme conditions," river management executive director Andrew Reynolds said in a statement.
"While some water is available for the Murrumbidgee, the drought and low inflows mean water storages are too low to increase flows in many other parts of the basin."
Commonwealth Environmental Water Holder Jody Swirepik says work is underway to bring together the small amounts of water available to improve the conditions and help native fish survive.
The release will include water from multiple water holders including the CEWH, the Living Murray initiative which is coordinated by the basin authority, and the NSW Office of Environment and Heritage.
It follows two mass fish deaths in the Darling River at Menindee in January with the NSW Department of Fisheries warning of more fish kills this summer.
"It's a spiritual part of who we are, we are river people,” Mr Kennedy said.
“This is killing our culture.”