AUSTRALIA is praying for rain as it faces the stark truth that the drought crisis is just six weeks from becoming an unprecedented disaster.

Murray-Darling Basin irrigators in four states will be denied water from July if there no substantial rainfall by June, with Prime Minister John Howard delivering an edict that urban water supplies must be protected, even at the expense of thousands of farmers facing a doubtful future under a worst-case scenario.

Adelaide residents are likely to share the pain as the State Government prepares to introduce extreme level five water restrictions if flows into the river system do not drastically improve. And all Australians face higher supermarket prices for the fruit and vegetables produced in the basin, which supplies 40 per cent of the nation's agricultural produce.

Mr Howard called an urgent press conference in Canberra yesterday to announce the drastic situation, saying a ''contingency planning report'' on water reserves had revealed the worst news yet during the harshest drought in history.

Mr Howard said the Government had no choice but to protect ''critical urban water supplies'' along the system, including that of Adelaide.

''You are simply not going to have enough water, consistent with the obligation to supply critical human needs to town communities along the river system, you are not going have enough water to provide any allocation for agriculture,'' he said.

Farmers face up to the dead dry

Mr Howard warned that without exceptionally heavy rains in the next six weeks, there was no prospect of water being available for anything other than urban use and both crops and stock would have to go without.

Long-range weather forecasts suggest a 50 per cent chance of better than average May rains in the Riverland but not for the rest of the basin.

Adelaide relies on the River Murray for about 90 per cent of its water supplies. The city now is on level three restrictions.

Farm representatives yesterday suggested Adelaide residents would have to ''do more'' and adopt tighter restrictions. Federal Water Resources Minister Malcolm Turnbull said Adelaide's supply of household water was safe for now but he offered no guarantees into the future.

''Yes it is, for this year. The officials are confident they can get enough water down to Adelaide,'' he said.

Asked how much rain was needed to avoid the zero irrigation scenario, Mr Turnbull declined to nominate a figure.

''A lot . . . I would suggest you just pray for rain, don't pray for a specific number of millimetres,'' he said.

Opposition water spokesman Anthony Albanese said the situation was made worse because the Government had failed to address climate change.

''This water crisis has not occurred overnight,'' he said.

''The water crisis has developed over a number of years and it shouldn't have taken an election year to get action from the Howard government over the issue.''

The Irrigation Association of Australia's chief executive, Jolyon Burnett, said scarcity could drive up food prices as irrigators rationalised their operations.

''Perennial crops, fruit trees for example, some of the growers are already watching and deciding which trees to let go, which parts of the orchard to shut the water off to,'' he said.

The Murray-Darling Basin spans fours states and the ACT and is home to about 50,000 stone fruit and citrus farmers, who contribute more than a $1 billion a year to the economy.

National Farmers' Federation water taskforce representative Laurie Arthur said the situation was unprecedented.

''We've never seen the like of this ever,'' he said.

''We'll be seeking urgent discussions with the Minister for Water and the Prime Minister to make sure that industry expertise can't find some way to try and protect some of the valuable assets we do have.''

The State Government meanwhile has warned that action will be taken against irrigators taking Murray water without permission.

South Australian Farmers Federation president Wayne Cornish said he did not think any farmers would break the law but some would be tempted.

''If your trees were dying and your vines were dying of thirst, there would be a fair temptation, I imagine,'' he said.