Mr Garrett has nominated a section of the river stretching from its junction with the Darling River near Mildura to the sea for conservation assessment.

If the river were found to be threatened ecologically, it could be listed as a site of national environmental significance under the Environment, Protection, Biodiversity and Conservation (EPBC) Act.

Such a move would oblige the minister to assess, and possibly block, developments on the river.

This could involve overruling private interests and the Victorian, NSW and South Australian governments.

The entire river below Wentworth, including the lower lakes and the Coorong, has been nominated for assessment, together with associated wetlands, flood plains and groundwater systems.

To be accepted for listing, a continuous part of the riverine environment would need to be found to be threatened.

However, a spokesman for Mr Garrett said the assessment would not be completed until 2011.

"Importantly, listing would not affect existing land use and the EPBC Act would only be triggered were there actions planned that would have a potentially significant impact on the ecological community," the spokesman said.

Existing irrigation practices would not be affected.

Rob Fowler, professor of environment law at the University of South Australia, welcomed the move.

"It would be another way of highlighting the crisis state of that system and attracting some additional protection for it," Professor Fowler said.

Under the act, the minister will have to assess the merits of a weir proposed to separate the Murray from the lower lakes, because the lakes' status as wetlands listed on the Ramsar convention on wetlands of international importance already triggers the EPBC Act.

The Federal Government and Opposition yesterday supported the establishment of a Senate committee to propose ways of saving the drought-stricken lower lakes.

"What a wonderful day that we've put the plight of the lower Murray right at the top of the political agenda," said Greens senator Sarah Hanson-Young, who sponsored the formation of the committee together with South Australian independent senator Nick Xenophon.

The committee is due to make its recommendations in a month.

Mr Garrett requested the assessment of the conservation status of another 26 animals, plants and habitats as well as that of the Murray.

These include the Tasmanian devil.

It is currently listed as vulnerable, but may be reassessed as endangered.