In the town of Roma, further north, a woman whose car was swept from a flooded roadway became the disaster's first official victim, with searchers finding her body two days after she went missing.

"Residents of St George are required to evacuate by road before the Moonie Highway is cut by floodwaters, which is expected to occur sometime in the early part of Sunday evening," police said in a mandatory evacuation order.

"Air evacuation to Brisbane is being organised to ensure evacuations can proceed after the Moonie Highway closes."

Police doorknocked every home in town to deliver the order and are permitted to use reasonable force if people refuse to leave, with mayor Donna Stewart warning resident safety could no longer be guaranteed.

"It's pretty panicky here," said St George resident Angela Doran.

"I think everyone just wants to get out now."

The swollen Balonne River, flooding for the third time in less than two years, was expected to peak at 15 metres (50 feet) on Tuesday night, far exceeding the previous record of 13.4 metres set in March 2010.

"This is a very serious situation and I urge people to cooperate," said Queensland Premier Anna Bligh.

Prime Minister Julia Gillard dispatched a military transport plane to airlift patients from the town's hospital to the coast, and the local nursing home was also being emptied.

Eight defence helicopters are already helping search and rescue efforts in the flood zone, winching people to safety and dropping food, bedding, medical supplies and other essentials such as generators.

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.

Deputy Queensland police commissioner Ian Stewart said the situation in the region had "deteriorated" overnight with "very large spikes" in water levels south of Roma, where 200 homes flooded earlier this week.

The woman who died in Roma had managed to save her seven-year-old son from the floodwaters but was swept away before she could be rescued, according to the Courier Mail newspaper. She had been helping neighbours to sandbag.

The Bureau of Meteorology said St George was expected to reach the 2010 flood level by Monday morning and "continue rising, possibly above 14 metres on Tuesday and Wednesday."

Residents scrambled to fortify levees still standing from last year's floods, with some locals saying they were prepared for a peak of 14.5 metres or worse.

"If it does reach the peak that they're talking about, if it goes across into the irrigation area, it's going to knock out thousands of hectares of cotton," local farmer Alex Benn told Sky News.

Flood defences held overnight in nearby Charleville, where some 600 people were holed up in an evacuation centre waiting for the Warrego River to subside.