The Morrison government will oppose an explosive Nationals rebellion aimed at dramatically shaking up the Murray-Darling Basin Plan.
Prime Minister Scott Morrison is being urged to strip the junior coalition partner of the water portfolio after its senators launched a push to change government legislation.
The proposal would ban the government from buying water from irrigators to return to the environment.
It would also block delivering 450 gigalitres of water, which most benefits South Australia, already earmarked for the environment.
The amendments also prevent any extra environmental water being allocated after the plan is completed in 2024.
Government Senate leader and SA Liberal senator Simon Birmingham said he was committed to the plan being delivered in full and on time.
"When those amendments come to a vote, I and the government will be voting against those amendments," he told parliament.
"The government stands resolute in its support for the implementation of the Murray-Darling Basin Plan."
Victorian senator Bridget McKenzie, who is tipped to return to cabinet as early as this week, is leading the charge for the changes.
Senator McKenzie denied Barnaby Joyce's return as Nationals leader was linked to the push for major changes to the basin.
"You'll never ever be able to come back into our communities and take water," she told parliament.
Crucial crossbench senator Rex Patrick threw down the gauntlet to the coalition in a fiery parliamentary debate.
"I will block my phone from every minister in the federal government. I won't be talking to them about any legislation," the South Australian independent said.
"They can play Russian roulette if they want."
Labor, the Greens and Senator Patrick are demanding Mr Morrison take water responsibility off the Nationals.
"Until the Liberal Party decide they are going to stand up to this sort of vandalism, you will be condemned," Opposition Senate leader Penny Wong said.
Nationals MP Damian Drum said the policy, which opens up another split with the Liberals, had been agreed in the party room.
"We are putting a line in the sand right now," he said.
He claimed environmental water was being wasted while more was being drawn from agriculture.
The Nationals are trying to tack the changes on to a bill giving substantive powers to the inspector-general of the basin.
The proposal could be scrutinised by a parliamentary committee before it comes to a vote.
Greens senator Sarah Hanson-Young accused Mr Joyce and his supporters of wanting to steal more water from the environment.
She challenged government Senate leader Simon Birmingham, an SA senator, to stand up to the Nationals.
"Stare down this wacky, crazy, untrustworthy mob and make sure they do not get their mitts on the portfolio, the public money and any more of the water," Senator Hanson-Young said.
South Australian Water Minister David Speirs has contacted his federal counterpart Keith Pitt - a Nationals MP - about his disappointment with the "stunt".
"The Marshall Liberal government categorically rejects the amendments put forward in the Senate," he told AAP.
Australian Associated Press