Greens Senator Rachel Siewert said she was "appalled" that Mr Garrett approved a project that "dramatically increases the amount of water extracted from the system".

"The city of Melbourne has much safer and easier options to improve its water security," she said.

"With the city of Adelaide now looking to wean itself of its dependence on the Murray it is ludicrous for Melbourne to increase its take."

The Greens are highly critical of the manner in which the environmental impact assessment for the pipeline has focused on the direct impacts on ecosystems in the path of the pipeline, without factoring in the consequences of removing more water from the Murray and of the additional greenhouse gasses created by the huge amount of energy required to pump this volume of water.

"Farming communities in the basin are already doing it tough and irrigators are struggling to get by with little or no water allocated and very high prices on the temporary market," said Senator Siewert.

"The audit report on the Victorian food-bowl project showed that there was no basis for the volumes of water it claimed it could recover through efficiency measures, and the Minister's assessment appears to have simply taken these rubbery figures at face value."

The Coalition also slammed the "bizarre decision" which they say will drain more water from the Murray than the Government is able to recover from buying irrigation properties like Toorale Station.

The Coalition says Mr Garrett also needs to explain why he is "ignoring a Senate Inquiry which asked the Minister to hold off making any decision on the pipeline until their investigation is completed".

The Standing Committee on Rural and Regional Affairs and Transport is currently examining the water management of the Lower Lakes and Coorong.

Any decision to take 75-110 billion litres of water a year from the Murray will clearly have an impact on flows into the Lower Lakes region in South Australia, the Coalition says.