Australia braced for a rapidly rising death toll Tuesday after flash floods killed eight and left 72 missing, as a quickly spreading flood disaster forced evacuations in central Brisbane.

A sombre Prime Minister Julia Gillard, dressed in black, warned the country to prepare for the worst after flash floods described as an "inland tsunami" smashed mountainside Toowoomba, sweeping away 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 death 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."

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.

Nineteen 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.

Meanwhile floods that have devastated an area the size of France and Germany combined threatened central Brisbane, the state capital, forcing evacuations in a riverside inner city area and warnings for a swathe of suburbs.