CANBERRA - Thousands of people have been isolated or evacuated as floods continue to inundate vast tracts of eastern Australia, with more on the way.

Pouring across land ravaged by a decade of the worst drought in history, floodwaters have wiped out hundreds of millions of dollars of summer crops, overwhelmed towns and houses, and closed major transport routes.

With waters still rising to critical levels along the levees and sandbagged sides of overflowing river systems, weather forecasters warned that heavy rain was again expected to dump across saturated states tomorrow.

Yesterday, New South Wales Premier Kristina Keneally added six shires to the list of natural disaster areas, bringing to 34 the number declared since rain-swollen rivers began bursting their banks in October.

Thousands of people have been forced to leave their homes, most in the southwestern regional centre of Wagga Wagga - where rivers were expected to peak late yesterday - and Coonamble, on the Castlereagh River in the state's northwest.

Many will not be able to return until at least the end of the week, and NSW Emergency Services Minister Steve Whan said yesterday that about 3000 people remained isolated.

Almost 70 people have been rescued from rising waters, including drivers trapped in their vehicles. Whan warned of the dangers of trying to cross floodwaters: "It can cause deaths."

Emergency service volunteers have been joined by soldiers and firemen, using bulldozers and sandbags to build defences around vulnerable towns.

Key roads running through NSW have been closed.

In Canberra, rain in the past week has already topped the average for December.

To the north, Queensland is also expecting more flooding, although less serious than in NSW or the earlier floods that swept through Victoria.

Flood warnings have been issued for the 150,000sq km of the Fitzroy River system sprawling across central Queensland, for the Balonne system feeding into the Murray Darling Basin in the state's southeast, and for other systems in the southwest.

Rain has also caused problems in Noosa, on the Sunshine Coast, Brisbane, and on the Gold Coast.

For farmers, the wild swing from drought to flood has been devastating.

As the drought broke, forecasters predicted record grain harvests in NSW, Victoria and South Australia - but many farmers now face the loss of their crops, or serious damage.

In NSW, official warnings predict that crop damage could top A$500 million ($619 million), but grain growers fear it could be far worse, possibly reaching A$1 billion.