Australia suffered an "inland tsunami" as flash floods killed nine and left 66 missing, and the city of Brisbane was braced for disaster Tuesday with 6,500 properties under threat.

A sombre Prime Minister Julia Gillard, dressed in black, warned the country to prepare for more deaths after the flash floods smashed the mountainside town of Toowoomba, sweeping away cars and entire houses.

"Yesterday we saw some simply shocking events in Toowoomba and other communities in the Lockyer Valley, literally walls of water smashing into cars and into buildings," Gillard said.

"We have seen very dramatic images of cars tossed around, people on roofs of houses and on the roofs of cars and people literally hanging on for dear life to trees and to signposts."

Queensland state premier Anna Bligh said the toll would rise "potentially quite dramatically", with families among those missing and rescue efforts hampered by heavy rain and washed-away roads.

"Mother Nature has delivered something terrible in the last 48 hours but there's more to go and our emergency people are more than up to that task," said Bligh.

"This is going to be I think a very grim day, particularly for the people in that region, and a desperate hour here in Queensland."

Twenty people have now died in flooding across Australia's northeastern coal-mining and farming zone after weeks of rain blamed on the La Nina weather system, which has also dumped heavy snow on the northern United States.

TV images showed Toowoomba's streets turned into churning rapids dotted with floating cars, some with people sitting on top, while elsewhere residents were forced onto roofs as waters lapped at awnings.

Four of the dead were children, some of them swept away in cars driven by their mothers. A man and a younger male died in Murphy's Creek near Toowoomba, 125 kilometres (80 miles) west of Brisbane in the Great Dividing Range.

Toowoomba mayor Peter Taylor said the flood struck without warning after two normally placid waterways suddenly overflowed, telling the Seven Network that "people had no warning at all".

"It was just unprecedented. Some people are saying an inland tsunami, and I think that probably sums it up really," he said.

Federal MP Ian MacFarlane described dramatic scenes in Toowoomba as the flash flood deluged the town before subsiding within three hours, leaving scenes of destruction and people dead in their cars.

"We're just seeing building after building, the water rushing in and blowing the windows out," MacFarlane told Sky News. "Cars that were parked in the car parks were just lifted up and went bobbing down the street."

Four military helicopters were sent to join the emergency effort but rescuers were badly hampered by continuing heavy rains in the Lockyer Valley region.

Meanwhile floods that have devastated an area the size of France and Germany threatened to bring the worst floods in decades to Brisbane, Australia's third biggest city, and swamp the central business district in coming days.

City mayor Campbell Newman said unrelenting rains would see a large-scale disaster hit the Queensland state capital on Thursday if the Brisbane River peaks as projected, affecting as many as 14,900 people.

"So the situation is very serious," he told reporters.

"Today is very significant, tomorrow is bad, and Thursday is going to be devastating for the residents and businesses concerned."

Thousands of people streamed out of the city centre in buses, trains and cars as the Brisbane River, which runs through the bustling eastern hub, burst its banks and officials issued flood warnings for dozens of districts.

Nearby, hundreds of people were air-lifted out of outlying towns as floods that have already cost billions of US dollars in damage spread yet further.