Thousands of people fled central Brisbane today as the panicked city braced for its worst flooding in 120 years, after terrifying flash floods left 10 dead and 78 missing nearby.

Workers evacuated en masse after the Brisbane River burst its banks and officials said 6,500 properties and the downtown area would be hit in Australia's third largest city, which was facing its biggest floods since 1893.

"This water is on its way," said Brisbane Lord Mayor Campbell Newman.

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

The exodus by bus, train and car came a day after flash floods devastated the hillside town of Toowoomba, 125 kilometres (80 miles) west in Australia's Great Dividing Range, sweeping away houses and uprooting trees.

In Brisbane, panic-buying took hold as flood alerts were issued for dozens of suburbs and rescuers air-lifted hundreds of residents from outlying towns.

"At our local supermarket queues, all 12 tills were at least 50-60 people deep and all essentials were sold out -- perishables and non-perishables," said Brisbane lawyer Paul Betros, who was sent home when his central office building was evacuated.

"There was no bread, milk, batteries, bottled water, candles. The bakery had sold out of bread and was closing."

The sudden loss of life in Toowoomba dramatically escalated a rolling flood disaster that has inundated vast swathes of Australia's northeast and crippled economic life.

The overall death toll now stands at 21 while three-quarters of Queensland state has been declared a disaster zone as a result of the relentless rains.

An emotional Queensland state premier Anna Bligh said more deaths were expected from the flash floods as rescuers, battling heavy rains and washed-away roads, reach cut-off communities.

"We are now in a very frightening experience," she said. "Can I appeal to everybody that it's at times like this that we all need to make an effort to stay calm, to be patient and stick together."

Prime Minister Julia Gillard, dressed in sombre black, called images of the disaster "simply shocking".

"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," she said.

TV pictures 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.

Five 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.

Bligh said the death toll from the Queensland floods had risen to 10 with the death of a four-year-old boy who fell out of a boat, according to the AAP newswire.

"This boy's life was lost during an attempt to rescue him and his family," Bligh told reporters, according to AAP, adding that the number of missing had risen from 59 to 78, with grave concerns for 15 people.

As Brisbane river neared its flood peak the city port late Tuesday was closed to ships in all but "emergency situations", Queensland Emergency Service Minister Neil Roberts was quoted by AAP as saying.

The city's airport, however, was expected to remain open throughout the crisis, airport authorities told the newswire.

"Biblical" flooding across the coal-mining and farming belt follows weeks of rain blamed on the La Nina weather system, which has also dumped heavy snow on the northern United States.

Federal MP Ian MacFarlane described fearsome scenes in Toowoomba as the flash flood deluged the town before quickly subsiding, 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."

Toowoomba mayor Peter Taylor told the Seven Network that the flood struck without warning after two normally placid waterways suddenly overflowed, saying "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.