The Greens and the coalition are at loggerheads over changes to the Murray Darling Basin Plan, as two states threaten to walk away if the federal government doesn't get its way.
NSW and Victorian Water Ministers, Niall Blair and Lisa Neville, said they would pull out of the basin plan if the Turnbull government's proposed changes were not passed in the Senate this week.
"There is a very real and imminent danger that Victoria and NSW, probably followed closely by Queensland, would pull out," federal assistant water minister Anne Ruston told reporters in Canberra on Tuesday.
"Playing political games just because we've got a South Australian election and putting this plan in jeopardy is a very serious thing to be doing."
But Greens senator Sarah Hanson-Young said Victoria and NSW were "trying to blackmail" the Senate.
"This is beyond pathetic now," she said.
"We are sick and tired of upstream big corporate irrigators ripping off our river and leaving us high and dry."
The Greens have two disallowance motions on the Senate notice paper that could block changes to the north and south basins of the Murray Darling due to be voted on this week.
The government is facing opposition from Labor, the Greens and the crossbench on the issue.
The proposed changes to the way the water of the Murray Darling is distributed takes the form of regulation, which means it can be disallowed by the Senate.
Senator Ruston called for Labor to be "reasonable and rational" and to support the government's changes.
"It would be disappointing if they don't support it, but we will remain at the table with (Labor's) Tony Burke in the hope we can get him to support them," she said.
"Most of the crossbench have been very receptive to the argument that the most important thing is to deliver the plan."
At a party room meeting on Tuesday, the Labor caucus agreed to continue to support the disallowance motion unless certain conditions were met before the vote.
Labor's concerns include responses to allegations of corruption, the accuracy of data and modelling, indigenous consultation and that taxpayer-funded environmental water is used for environmental purposes.
Senator Hanson-Young said if Labor flipped and supported the government changes it would "go down like a lead balloon in South Australia."
"If Labor holds firm... if they hold tight we should be able to get this through."
