The $13 billion Murray-Darling Basin Plan aimed to return 450 gigalitres of water to the environment by June 2024.

The government has put forward legislation to move that to the end of 2027.

Environment Minister Tanya Plibersek announced the amendments in exchange for crucial support, at a joint press conference with Greens senator Sarah Hanson-Young on Monday.

Amendments include an independent audit of water throughout the basin, and $100 million in funding for the Aboriginal Water Entitlement Program.

The proposed laws will be introduced to the Senate this week, with the government negotiating with other crossbenchers to get the numbers needed for the bill to clear the upper house.

The changes will put into place a new agreement with all basin states, except Victoria, after it was revealed water recovery targets would not be met.

The bill will now guarantee that the 450 gigalitres will be recovered by 2027.

Without the support of the Greens, an end of year deadline would be missed, and the government would be forced to start water buybacks for about 300 gigalitres of water next year.

Plibersek could not say how much water buybacks would cost the taxpayer.

“It is inconceivable that we fail the environment and fail in land communities again as a parliament,” she said.

“If this legislation doesn’t pass this week, we will go into a range of automatic timelines that come into play under the Murray-Darling Basin Plan, wouldn’t be good for farmers … and it will be a disaster for the environment.”

Plibersek said water will be bought back voluntarily from both the north and south parts of the basin, and described buybacks as “unavoidable”.

The environment minister said with the timeline extension, more water saving projects can now be delivered, which would have otherwise been withdrawn.

Hanson-Young said for more than a decade the basin had been “rorted”.

“The Murray-Darling Basin Plan has not delivered what it promised to deliver,” she said.

“These amendments and the commitments that we’ve been able to negotiate with the government today will restore health to the river and trust in the plan.”

The coalition wanted the government to include a social and economic impact test for the recovery of the water, and to keep the cap on buybacks.

Opposition water spokeswoman Perin Davey said the plan would cost households by pushing up grocery prices  there would be job losses in regional communities.

Davey said in time of drought there was no rain, so “it does not matter how much water you hold in licences because that water is not there”.

The next El Nino cycle means there will be less rain and drier conditions which are expected to impact the health of the basin.

The plan limits the amount of water taken from the basin to keep the river system sustainable.

– AAP