A giant inland sea of floodwater, 55 miles (90km) long, will spread across the Australian state of Victoria over the next 10 days, officials say.
Deputy Prime Minister Wayne Swan said the floods would rank as one of the most costly natural disasters in the country's history.
More than 30 people have been killed since flooding began last month.
In Queensland, nine people are still missing after floods tore through the cities of Toowomba and Grantham.
The Victoria State Emergency Service has issued evacuation warnings for communities east of the city of Kerang, which remains cut off.
In all, more than 70 communities have been affected. In the city of Swan Hill, people have been building makeshift levees to hold back the Murray River, which is expected to carry the bulk of the floodwaters as they run off over the next 10 days.
These are the worst floods in northern parts of Victoria since records began 130 years ago.
Sandbags and misery
"There is no doubt the recent floods will rank as one of the most costly natural disasters in our history," said Mr Swan, who is also Australia's treasurer.
The impact of the floods was worse than a series of natural disasters in the 1970s and wildfires in 2009 in which 173 people died, he said in his first economic note of the year.
Further north, in Queensland, residents of the state capital, Brisbane, have again been putting out sandbags as high tides threaten to inflict more misery on low-lying suburbs.
The city is still clearing up after floodwaters two weeks ago reached a peak of 4.46m (14.6ft).
The search for the bodies of flood victims is continuing.
The Australian navy has been trying to clear the Brisbane River of tonnes of debris including cars, parts of buildings and boats, says the BBC's Nick Bryant in Sydney.
The floods are expected to pose a threat for another week, our correspondent says.
Economists estimate that the flooding in Queensland and Victoria will cost at least 3bn Australian dollars (£1.8bn) in lost coal exports and agricultural production.
Reconstruction could cost an additional 20bn Australian dollars, the ANZ Bank says.
The Queensland Premier's Flood Relief Appeal has so far raised 135m Australian dollars.
