The federal takeover of the ailing Murray-Darling Basin is hanging in the balance after the Senate baulked at the plan.
The government wants to wrest water powers from the states and spend $13 billion conserving water in the drought-ravaged basin.
But opposition senators won't play ball, saying the takeover does not go far enough and demanding changes.
The government has rejected some of those changes, leading to a water stand-off which could jeopardise one of Labor's key election promises.
The coalition, Greens and independent senator Nick Xenophon have teamed up and issued a list of demands.
They want the controversial Sugarloaf pipeline, which will pipe 75 billion litres of the basin's water a year to Melbourne, to be scrapped.
They want a masterplan to cut water use to be in force well before the government's deadline of 2019, and are demanding more money be spent on waterproofing irrigation infrastructure.
And they say any new mines in the basin must not soak up much water.
Opposition water spokesman Greg Hunt said the government's water plan was not good enough.
"We have put together a historic coalition to save the Murray," he said.
Greens water spokeswoman Rachel Siewert said there was no point passing the government's soft water plan.
"It's not time to sit around and make small, incremental changes to the Murray," Senator Siewert said.
"We need a much more aggressive approach."
But the government has rejected some of the opposition senators' demands.
Water Minister Penny Wong accused her opponents of wasting time.
"If the opposition and cross-bench parties want rapid action, they should stop playing political games to delay that action," Senator Wong said.
She said the Liberals had promised to support the new water plan and they should keep their word.
Senator Wong also accused the Liberals of hypocrisy, saying they had failed to save the Murray-Darling in 12 years in government.
The Senate has amended the Water Amendment Bill to include the opposition's demands. The bill could pass the Senate on Thursday before going back to the lower house.
Meanwhile, lobby group GetUp! presented a petition of 47,000 signatures from people concerned about the Murray's future.