Tsunami-like flash floods raced toward Australia's third-largest city of Brisbane on Tuesday, prompting evacuations of its outskirts, flood warnings for the financial district and predictions that the death toll is likely to climb.

The worst flooding in the coal-exporting state of Queensland in half a century has killed 12, but that number is expected to rise, state premier Anna Bligh said in Brisbane, a city of two million people near the mouth of a large river.

"We have a grim and desperate situation," Bligh told a news conference, adding that the flood threat to state capital Brisbane would peak on Wednesday or Thursday.

In Toowoomba, a town west of Brisbane, eight people were killed on Monday night when a "super rainstorm" sent a 2-meter wall of water through streets on Monday, sweeping away cars and pedestrians.

"Early reports would indicate that what hit Toowoomba could best be described as an inland, instant tsunami, with a massive wall of water that's gone down through the Lockyer Valley," Police Commissioner Bob Atkinson said.

TV footage showed brown floodwater gushing through the center of Toowoomba laden with debris, as people clung to telephone poles and rooftops to survive.

Panicked motorists climbed onto cars to escape the deluge, which destroyed homes and bridges, and hurled cars into trees and buildings like corks.

Police warned people living in up to 32 low-lying Brisbane River suburbs and further west to head to high ground as a second day of torrential rain was expected to see rivers swell by up to 16 meters in height.

Sky TV reported people were evacuating from some parts of downtown Brisbane on Tuesday, but police told Reuters only some city outskirts were being evacuated.

"It's like an atomic bomb hit this place," Steve Jones, mayor of Lockyer, another town affected by the downpour, told the Courier Mail paper. "The intensity was impossible to explain."

Rivers in some areas rose more than eight meters in an hour, catching residents and authorities completely by surprise.

Police said more than 40 people were pulled from rooftops by helicopters in a desperate night operation that saw even media helicopters pressed into action. More than 70 people were missing on Tuesday.