Furious Victorian and NSW water ministers could be poised to pull their states out of the Murray Darling Basin Plan after federal government changes seeking to distribute water differently were shot down in the Senate.
The coalition was unable to strike a deal with Labor, which supported a Greens disallowance motion, with it passing 32 votes to 30 on Wednesday.
NSW Water Minister Niall Blair said the plan, which delivers healthy rivers and viable regional communities, had been thrown on the scrap heap in a "race for votes in South Australia".
"This move makes the basin plan untenable for NSW," he said.
His Victorian counterpart Lisa Neville has also flagged intentions to walk away.
"(It) is a slap in the face to communities and a slap in the face to the environment," she said.
"We want the commonwealth to investigate all options to overturn this decision. If that is not possible the plan cannot be delivered."
The federal government was attempting to reduce the amount of water being returned to the environment in southern Queensland and northern NSW, easing pressure on farmers in those regions.
"Make no mistake, tonight the Australian Senate puts the Murray-Darling Basin Plan in peril," cabinet minister Simon Birmingham told parliament on Wednesday.
Water Minister David Littleproud said Labor had now brought back the "water wars" between the states.
"Labor has killed the plan it worked on for years and had created huge uncertainty for thousands of businesses in regional towns which rely on water allocation," he said in a statement.
"After putting farmers and rural communities through painful uncertainty for several years, Labor does it again."
But South Australian Greens senator Sarah Hanson-Young said blocking the changes would provide the chance for integrity to be restored to the plan after allegations of water theft in the northern part of the basin.
She believes the changes will hurt communities who rely on the Murray River in SA.
Labor Senate Leader Penny Wong said until the government was serious about tackling the theft and allegations of corruption, the party was not in a position to support the northern basin proposal.
"It was Labor that delivered the Murray Darling plan in government and it is Labor that will fight today to save that plan, the whole plan, the entire plan, for all Australians," she said.
The National Farmers Federation said Labor's actions were at best short-sighted and reckless and at worst a "sign of contempt for regional Australia".
Water has been a hot issue in the South Australian election, where Labor is seeking to retain government.