Greens leader Bob Brown said he believed the inquiry would be better able to determine water storages than the independent audit of the Murray-Darling Basin promised two weeks ago by Prime Minister Kevin Rudd.

The motion was moved by new Greens senator Sarah Hanson-Young and supported by the Government, the Opposition and South Australian Independent senator Nick Xenophon.

It calls on the Senate's Rural and Regional Committee to report no later than September 30 on the volume of water and ways in which it could be provided into the Murray-Darling system to replenish the lower lakes and Coorong.

Senator Hanson-Young said, ''The multi-party support indicates the scope of this environmental crisis. It shows acceptance by all sides of politics that we have to act.''

The Senate also supported the Greens' motion for a second inquiry due to report on December 4, which will examine the implications on the long-term sustainable management of the Murray-Darling Basin system.

Senator Brown said the support by all parties was an ''early dividend'' for those Australians who supported a return to the Senate with a balance of power out of the hands of the two major parties.

''It is a very early indication of how a balance of power in the Senate can lead to very good outcomes,'' Senator Brown said.

He said the inquiry would have greater power than an independent audit and would also be able to, for example, examine water volumes in the Snowy Hydro Scheme.

Figures issued yesterday by the Australian Bureau of Statistics showed that irrigation water use fell by 29 per cent in 2006-07 to 7636 gigalitres. The largest decline was in water used for rice growing, which fell by 81 per cent, followed by a 50 per cent decline in irrigation water use for cotton growing and a 30 per cent fall in the amount of water used for pasture and grazing.

Earlier this month, the Australian Conservation Foundation proposed the Government buy six stations in the northern part of the Murray-Darling Basin in far-western NSW and Queensland to free up 300GL to increase water flows downstream.

But the plan was dismissed by Climate Change Minister Penny Wong who said 80 per cent of the water would not make it to where it was needed at the lower lakes.

Opposition water security spokesman John Cobb questioned the Government's sincerity over the water crisis.