VICTORIAN and NSW irrigators are set to gain access to one million megalitres of water due to flow down the Barwon-Darling River to the Menindee Lakes this autumn.

The NSW Government is to divert the Queensland floodwater into the largest of the Menindee lakes, Lake Menindee, rather than down the Darling River to South Australia's Lower Lakes.

The decision has been welcomed by NSW and Victorian irrigators, who say it will underpin their allocations for next season.

But in what is regarded as a bizarre political move, South Australian Premier Mike Rann yesterday tried to claim he had struck a deal to secure 400,000 megalitres of the Queensland water for South Australia's lower lakes.

The announcement shocked water officials in NSW, Victoria and even SA, who yesterday said they knew nothing of any deal.

Two top NSW and Victorian water bureaucrats said it appeared Mr Rann was simply laying claim to floodwater that would be stored in the Menindee Lakes and would eventually flow to SA but was fully under NSW and Victorian control.

"It's just part of SA's entitlement that's for their towns, irrigators and Adelaide, not for their lower lakes," a Victorian official said.

The officials said NSW and Victoria would ultimately decide when to release the floodwater stored in the Menindee Lakes in consultation with the Murray Darling Basin Authority.

"It appears to be another example of him (Mike Rann) claiming credit for the sun rising in the morning," one top state water bureaucrat said.

Mr Rann faces an election this Saturday, with opinion polls showing he is in a neck and neck race with the Opposition for his political survival.

But whatever Mr Rann may claim, the NSW Government's decision to fill Lake Menindee is set to further boost Victorian Murray irrigators' allocations, which rose to 78 per cent this week on the back of recent rain and earlier NSW Christmas-New Year floods that filled Lake Victoria.

It's still uncertain whether the Murray Darling Basin Authority will be able to assess the latest flood volumes in time for northern Victoria's last allocation for this season on April 1.

At this stage, it appears the MDBA assessment may not be completed until April 15, with the water instead factored into next season's allocations.

NSW and Victorian government water analysts say storing the water in Menindee Lakes all but guarantees carryover water and an opening allocations for NSW and Victoria next season.

Victoria and NSW governments will be able to use the water stored in the Menindee Lakes to meet their monthly flow obligations to South Australia for about six months, instead of tapping into their Hume and Dartmouth dam reserves.

Victorian Farmers Federation water council chairman, Richard Anderson, said the NSW Government's decision was fantastic news for all irrigators and Adelaide.

"Common sense has prevailed," Mr Anderson said.

"It would have been foolish to let it go down to South Australia's Lower Lakes, especially if things stayed dry in the southern basin."

Under the Murray Darling Basin Agreement the NSW Government must share half the Menindee Lakes' inflows with Victoria, once their combined volume hits 640,000 megalitres.

But until now the NSW Government has only filled two of the Menindee system's four lakes to keep them under the 640,000 megalitre trigger, ensuring it retained ownership of all lake inflows.

Keeping the lakes below the trigger also denied Victorian irrigators access to 200,000 megalitres they owned, but could not use, in the lakes.

The issue came to a head earlier this year when frustrated Victorian irrigators watched the NSW Government divert half the one million megalitres from the Christmas-New Year floods past the Menindee Lakes into its share of Lake Victoria on the Murray and on to South Australia.

But the massive scale of the most recent one million-megalitre surge from Queensland has convinced the NSW Government to store the water in the Menindee system's largest lake, Lake Menindee, for the first time since 2002.

The decision comes at a cost, as the NSW Office of Water estimates it will lose 110,000 megalitres in seepage and even more in evaporation filling the 250-square kilometre Lake Menindee.

But the alternative would have risked letting water flow down the Darling to the Murray River and South Australia's Lower Lakes, which loses 750,000 megalitres in evaporation annually.

NSW water commissioner David Harriss said the benefits of storing the Queensland floodwater in Lake Menindee easily offset the losses.

Mr Harriss said the water stored in Lake Menindee, and the other two lakes in the system, benefited NSW and Victorian irrigators and secured a reserve for Adelaide.

"It's a secure water reserve for all states," Mr Harriss said.

"It means there's more water available in the Darling for South Australia, which means water can be held back in the Hume and Dartmouth."

He said NSW and Victoria could now meet their monthly flow obligations to South Australia from the Darling, rather than dipping into their upper Murray reserves in Hume and Dartmouth dams.

While Goulburn irrigators gain no direct benefits from the Menindee inflows, they are likely to gain access to much cheaper water as the water is allocated to their counterparts on the Murray next season.