A proposed extension to a looming end of year deadline for the Murray-Darling Basin Plan is yet to clinch the crucial support of the Greens or coalition.
The $13 billion plan aimed to return 450 gigalitres of water to the environment by June 2024, but legislation put forward by the Albanese government would push that back to the end of 2027.
The laws would put in place a new agreement with all basin states, except Victoria, after it was revealed water recovery targets would not be met.
Environment Minister Tanya Plibersek needs the bill to clear the upper house or else the deadline will be missed.
This means the government will be forced to buyback about 300 gigalitres of water from next year, which is more than 120 Olympic-sized swimming pools.
Large water recovery projects that are under way won't be completed, as they will have to be withdrawn.
In exchange for their support, the Greens want the bill to guarantee the 450 gigalitres will be recovered.
But the coalition wants the government to include a social and economic impact test for the recovery of that amount of water.
Ms Plibersek said by voting against the bill, the Nationals would end water-saving infrastructure projects.
"Without those water infrastructure projects, the Commonwealth will instead be forced to buy more water, more quickly," she said.
"Basin communities deserve better than this betrayal from the Nationals."
Nationals leader David Littleproud said the previous Murray-Darling Basin plan that had been reached was "akin to getting peace in the Middle East".
"All that Tanya Plibersek has done in a nasty and vindictive way, is open up the trauma of these rural communities again when it didn't need to happen," he said.
A group of graziers, irrigators and Indigenous leaders visited Parliament House on Tuesday, urging parliamentarians to guarantee more water is returned than is taken out.
Gloria Jones, a multi-generational fisher from South Australia, said more science wasn't needed to achieve the plan.
"We know what needs to be done, minister Plibersek we ask you to be brave," she said.
National Farmers' Federation chief executive Tony Mahar said the process had been rushed.
"What we can't have is a government single-handedly deciding they are going to rewrite this basin plan," he said.
"We have to have co-operation, we have to have collaboration."
The environment minister has warned the new plan was needed as the next El Nino cycle meant less rain and a higher risk of bushfires.
The plan limits the amount of water taken from the basin and seeks to restore the health of the river system.
