SA Water Minister Karlene Maywald announced plans yesterday to buy more than 11 billion litres of water that SA irrigators originally tried to sell to the Commonwealth to aid the Murray.

The Commonwealth recently rejected attempts by some irrigators to sell water back to the Murray, because the current round of their $3.1 billion buyback scheme was over-subscribed.

The Commonwealth buyback scheme is being conducted in stages to ensure purchases are in line with the budget's forward estimates.

The unsuccessful irrigators were promised another chance at selling their water back to the river, with new buyback rounds set to begin within the next nine months. But Ms Maywald said SA would seek to get in first and buy the water for critical human needs - drinking supplies in towns and cities.

She said the move would bolster drinking supplies in 2010-11 and also help struggling farmers.

"We understand that many irrigators had factored water sales through the Commonwealth's buyback into their business plans and now as a result of the full subscription of the program, are uncertain about the future," she said.

Australian Conservation Foundation spokesman Dr Paul Sinclair said "critical human needs" was a vague term and he hoped the environmental needs of the Murray River remained a priority.

"If it's genuinely for critical human use we would have no problem, but we need to ensure the speed at which water is being purchased to help the Lower Murray is matching the scale of the environmental catastrophe that has unfolded there," he said.

Commonwealth spokeswoman Ilsa Colson said $790 million was spent last financial year buying back 507 billion litres of water entitlement.

She said the states were entitled to enter the water market as they saw fit for either critical human needs or the environment.

The Brumby Government has confirmed plans to divert about 12 billion litres of water that would usually be destined for the parched Wimmera-Mallee region, and instead send it to Melbourne through the north-south pipeline.

The diversion will be a one-off event in a bid to ensure Melbourne gets its promised 75 billion litres in 2010.

The water would have gone to rivers in the region, but the Government defended the move, saying nearby water-saving projects were completed years ahead of schedule and the water was available earlier than planned.