Flood waters rose Monday in parts of Queensland but residents of a threatened town in the Australian state were thrown a lifeline with news that the levee on a swollen river might hold.

Thousands of Australians have been forced to abandon their homes as a record deluge sweeps through areas still reeling from last year's devastating flooding, with St George, in Queensland's south, under most threat Monday.

Many of its residents fled Sunday evening to evacuation centres in nearby Dalby or the state capital Brisbane, although some 400 stayed to help limit the damage despite a mandatory evacuation order.

Local mayor Donna Stewart said the Balonne River in St George, flooding for the third time in less than two years, had reached 13.48 metres (44 feet) and was expected to keep rising until at least Tuesday night.

Forecasters earlier estimated it could top 15 metres, breaching the town's 14.5-metre levee, but they have now revised down the predicted peak to just over 14 metres, making it touch and go whether the levee will hold.

"It's not out of the question it'll go above the 14.5 level but it's more likely not to," said Bureau of Meteorology hydrologist Chris Leahy as authorities dumped mountains of dirt around town to shore up its defences.

Stewart said she was optimistic the town could survive the worst-case scenario.

"The picture is not as grim as what it was looking this time yesterday," she said.

State Premier Anna Bligh, who launched a flood relief appeal, said the St George disaster had been the largest ever evacuation of a town in Queensland.

"The heartbreaking job of calculating the loss is still in its early stages," she said.

Reports said about 30 houses and businesses had been inundated so far, and it was expected to rise.

While most residents have fled, Queensland Police Commissioner Bob Atkinson said officers would stay to prevent looting.

"We are going to do everything we can to make sure people's homes are safe," he told reporters.

Federal politician Barnaby Joyce, who lives in the town, said watching the flood crisis unfold was "a little bit scary" with the river raging near his home.

"There is something that sounds a little bit like the sea but it is not actually the sea, it's a river and it is just outside the back door," he told ABC radio.

St George has seen major flooding twice in the past two years, once in March 2010 and again last year during Queensland's flooding disaster, which claimed 35 lives and swamped vast tracts of farmland and tens of thousands of homes.

Flooding has been hitting parts of Queensland and New South Wales over the past week but has claimed just one life: a woman whose car was swept from a roadway in Roma, further north of St George.

In that town and nearby Mitchell, a mopping up operation has begun with state Police Minister Neil Roberts saying the scale of the damage was slowly emerging.

"Preliminary assessments show that in Mitchell 280 homes or more have been damaged with water above the floor boards," he told reporters.

"In Roma, it's at least over 400. That's very significant damage and a very significant dislocation for those individuals."