Irrigators, graziers, Aboriginal groups and environmentalists have urged the NSW government to stop allowing floodplain harvesting in the Murray-Darling Basin.
Floodplain harvesting is when the water that flows during a flood is collected and used elsewhere.
In an open letter to NSW Water Minister Niall Blair, the broad alliance say they oppose a plan to give away rights to divert huge volumes of water in the northern basin.
Left-leaning think tank The Australia Institute is behind the letter which says harvesting has been unregulated and unmonitored.
Southern Riverina Irrigators chair Chris Brooks took aim at the proposed monitoring methods which he says uses 19th century technology.
“This year’s cotton crop in the northern basin will need about three million megalitres of water,” he said.
“The majority of that water must have been captured off the floodplain, with no regulation, monitoring or control.”
Australia Institute senior water researcher Maryanne Slattery said the practice was hurting the environment.
“Floodplain harvesting is a major contributor to the fish kills and water crises that have appalled all Australians this year,” she said.
A federal government-commissioned report into massive fish deaths in NSW last month recommended greater transparency around floodplain harvesting.