Helicopter rescue crews are trying to reach hundreds of people trapped by rising floodwaters in Bundaberg in the Australian state of Queensland.
Many people are trapped on roofs, but there are fears homes could be washed away by fast-moving waters after the Burnett River burst its banks.
Three people have already died in the Queensland flooding, after a cyclone brought heavy rain.
Australian PM Julia Gillard offered her condolences to the families of victims.
Ms Gillard was speaking in Gipsland, Victoria, where she was visiting people afflicted by the recent wildfires.
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Do not wait. Move now. Yr life depends on it”
Ian Stewart on Twitter Queensland police commissioner
She said it had been a "tough period" for Queensland, and that the whole country was being "challenged by nature".
"But we are a strong and smart nation and we'll get through this, as we always do, by pulling together," she said.
Bundaberg Mayor Mal Foreman said the floods would be "far worse" than those of late 2010 and early 2011, which left 35 people dead across Queensland, with Bundaberg among the towns affected.
In 2010, the Burnett River at Bundaberg reached 7.92 metres, but at 15:00 (05:00 GMT) on Monday, the river was at 9.2m and rising slowly, the Bureau of Meteorology (BoM) said in a statement.
It is expected to peak late on Tuesday or early Wednesday. Major flooding has also been reported at Mundubbera and Gayndah, said the BoM.
Residents of North Bundaberg were ordered to evacuate immediately, with the state's Police Commissioner Ian Stewart tweeting: "Do not wait. Move now. Yr life depends on it."Police Superintendent Rowan Bond said many people had only a few hours left in which to leave, before becoming stranded, saying there was an "imminent danger of people being killed and drowned".
But the waters are moving too quickly for boats to be used, and there are fears that homes could be swept off their foundations by the torrent.
At least 1,200 properties have flooded and State Premier Campbell Newman said up to 1,500 people could still be in need of rescue.
"We've never seen floodwaters like this before," he told reporters.
"People probably feel, quite reasonably, that they've seen this all before and they feel they can be safe in certain parts of North Bundaberg. Right now our view is they are not, and that's why we need to get them out.
Mr Newman said 14 helicopters were already at work plucking people from rooftops but that more were being brought in. He urged stranded people to group together and ensure less mobile people were not left behind.
"I can assure people who may be in harm's way at the moment that we are doing everything we can to come to get you to get you out of this situation," said Mr Newman.
By late afternoon, the main Bundaberg evacuation centre at Oakwood State School was itself evacuated, the Brisbane Times reports. Some 300 people were being moved to another centre near the airport.
Elsewhere in the state, some 350 homes have been flooded in Ipswich. More than 200,00 people are without power across the state.Residents of the Lockyer Valley, which was the scene of deadly flash floods in 2011, have also been told to evacuate as creeks and rivers rise.
The town of Gympie has been cut off and dozens of businesses are underwater, ABC News reports, while hundreds of homes are threatened in Ipswich, where the Bremer River is expected to peak later in the day.
Police said the body of an elderly man who went to check on a yacht north of Bundaberg had been recovered on Sunday. Two men swept away by flooding in separate incidents in Queensland were also found dead on Monday.
In Brisbane, which was paralysed by flooding two years ago, water has been creeping into low-lying parts of the central business district.
A woman and her three-year-old son were also taken to hospital in Brisbane after a tree fell on them.
The rain was brought by Tropical Cyclone Oswald, which is now affecting the northern part of New South Wales.
The BoM has warned of heavy rain, usually high tides and damaging winds, with gusts of up to 100 km/h (62 mph). Flash flood warnings have also been given.
A number of roads have been closed in the state and 6,000 homes are without power in the north.