Australia's flood disaster worsened today when at least four people died as a torrent of water swept cars and pedestrians into churning rapids.
Severe downpours deluged the nation's already sodden northeast, swelling rivers to fresh peaks and submerging roads and bridges, causing flash flooding in Toowoomba west of Brisbane, Queensland Premier Anna Bligh said.
"Mother Nature has unleashed something shocking on the Toowoomba region," Bligh told reporters. "This is without a doubt our darkest hour of the last fortnight."
Queensland police said four people, including two children, had died but that a number of others were possibly still missing in the surging brown waters which have swamped an area the size of France and Germany.
Bligh described the waters as "a freak of nature", saying they hit communities with "lightning speed" and left at least six people missing.
"We believe we're looking for at least three young pedestrians and two vehicles that seem to have been washed away," she said.
Footage showed vehicles flipped and tossed into trees in Toowoomba when the sudden torrent swept through.
"A wall of water came down, picked my ute (utility vehicle) up, pushed it sidewards," said Lockyer Valley mayor Steve Jones, likening the damage to a cyclone or atomic bomb.
"If it had been a little car it would have killed everyone in it."
Houses were ripped from wooden foundations and carried hundreds of metres by rushing water, which also slammed cars into shops and uprooted the local service station's petrol pumps, Jones said.
Landslides hit a major highway, bridges were swept away and a building reportedly collapsed in a deluge that Toowoomba councillor Joe Ramia said was unlike anything he had ever seen.
"I've seen streets and football fields... that are just oceans of water," Ramia told AAP news agency.
"Cars were floating, they've got streets barricaded off.... I've seen streets in Toowoomba that I never, never thought would carry that much water. It was just horrendous."
Bligh said authorities held grave fears for the safety of a number of people in the nearby Grantham township.
"We are unable to reach some 30 people in the town of Grantham," she said. "They have all gathered together in a primary school at Grantham. They are completely isolated by fast moving flood waters."
The premier said the military would begin searching for stranded people at first light while police issued warnings that residents of low-lying areas of other towns including Chinchilla should evacuate to higher ground.
Elsewhere, the town of Gympie was cut in two by the surging water while parts of Dalby, flooded four times already in the crisis, were again under water, forcing traumatised residents to evacuate ahead of another river peak.
Earlier Monday sandbags were handed out in Brisbane, the Queensland state capital of two million people, as the severe weather threatened to swamp low-lying areas and submerged some bridges and roads in the surrounding valley.
Prime Minister Julia Gillard said 150 regions across three Australian states had been hit by the deluge since November. More than 8,000 claims for emergency assistance had been made, worth Aus$10 million, and many more were expected.
But the damage bill would take some time to process, Gillard warned, with the flooding's end not yet in sight and many road, rail and bridge assets still under water.
The deluge has wiped out crops and brought dozens of coal mines to a standstill, driving up world prices and causing problems for the key steel-making industry.
The disaster, which has now claimed more than a dozen lives, is expected to shave at least Aus$6 billion ($6 billion) from Australia's economy.