(AP) -- Military helicopters searched Tuesday for scores of people missing after a tsunami-like wall of water ripped through an Australian valley, tossing cars like toys in the deadliest episode of a weekslong flood crisis.
At least 10 people were killed and 78 still unaccounted for almost 24 hours after the flash flood hurled untold millions of gallons of water down Queensland state's Lockyer Valley on Monday, state Premier Anna Bligh said. Authorities had grave fears for at least 18 of the missing, she said.
The valley funneled rain from a freak storm - forecasters estimated up to 6 inches (150 millimeters) fell in half an hour near Toowoomba city - into a stream that formed a path of destruction, lifting houses from foundations.
The torrent slowed and spread out as it moved downstream toward the state capital of Brisbane, Australia's third-largest city with some 2 million people.
The Brisbane River overflowed its banks Tuesday and officials warned that thousands of houses in dozens of low-lying neighborhoods and parts of downtown could be inundated by Thursday. Residents queued for up to four hours outside emergency services depots on Tuesday to get sandbags to try to protect their homes, and shoppers stocked up on bottled water, milk and fuel. Residents in at least three suburbs were asked to prepare their homes, then go and stay with friends or family on higher ground.
"This is a truly dire set of circumstances for the people of Queensland, with more flooding to come," Prime Minister Julia Gillard told Australian Broadcasting Corp. television. She said she had been "absolutely shocked" by television footage of Monday's flash flood.
The violent surge near Toowoomba on Monday escalated Australia's flood crisis in Queensland state and brought the overall death toll to 20. Until then, the flooding had unfolded slowly as swollen rivers burst their banks and inundated towns while moving downstream toward the ocean.
Emergency services officers plucked more than 40 people from houses isolated overnight by the torrent that hit the Lockyer Valley, and thousands moved out of were being evacuated. In one small community, Forest Hill, the entire population of about 300 was airlifted to safety in military helicopters, Bligh said.
In Ipswich, a town of 15,000 people between Toowoomba and Brisbane, hundreds of residents moved in with friends and family or into an evacuation center on high ground as officials warned the swollen Bremer River would flood dozens of homes overnight Tuesday, Mayor Paul Pisasale said.
As Tuesday progressed, the death toll rose from eight to ten. At an evening afternoon news conference, Bligh said officials held grave fears for 18 of the 78 people missing. She did not elaborate.
The search and rescue effort was being hampered by thunderstorms and more driving rain, though the bad weather eased during the day and Bligh said the search would get easier on Wednesday.
Bligh said about 9,000 homes in Brisbane could be badly affected, and tens of thousands more could have floodwaters enter their backyards. The flood peak was expected on Thursday, when parts of downtown were expected to be awash.
The city is protected by a large dam built upstream after floods devastated downtown in 1974. But the reservoir was full, and officials had no choice but to release water that would cause low-level flooding in the city, Mayor Campbell Newman said. The alternative was a much worse torrent.
Bligh said five children were among those killed and that many of those still stranded or unaccounted for are families and young children. On Tuesday morning, she said the death toll was eight and the missing numbered 72, and revised those numbers later. The most recently recorded death was that of a boy, who was swept away in the floodwaters.
Queensland has been in the grip of its worst flooding for more than two weeks, after tropical downpours across a vast area of the state covered an area the size of France and Germany combined. Entire towns have been swamped, more than 200,000 people affected, and coal and farming industries virtually shut down.
"The power of nature can still be a truly frightening power and we've seen that on display in this country," Gillard told reporters earlier in Canberra.
The floods reached a second state on Tuesday. Floodwaters bloated by driving rain in recent days cut roads to several towns in northern New South Wales state near the border with Queensland. About 4,500 people were cut off, the State Emergency Services said, though the situation was not yet as dire as in Queensland.
Monday's flash flooding struck without warning in Toowoomba, a city of some 90,000 people nestled in mountains 2,300 feet (700 meters) above sea level. Bligh said an intense deluge fell over a concentrated area, sending a 26-foot (eight-meter), fast-moving torrent crashing through Toowoomba and smaller towns further down the valley.
Police Commissioner Bob Atkinson described the events Monday as "an inland instant tsunami."
As the water was pushed its way downstream, officials closed roads and highways and told residents in low-lying area of Brisbane to sandbag their homes and then move to higher ground.
"We have a grim and desperate situation," Bligh said. "This took everybody so unawares that there was no opportunity in most cases for people to get to safety."
Heavy rain continued Tuesday, and the Bureau of Meteorology warned that more flash floods could occur.
Deputy Police Commissioner Ian Stewart said rescue efforts were concentrated on towns between Toowoomba and Brisbane, including hardest-hit Murphy's Creek and Grantham, where about 30 people sought shelter in a school isolated by the floodwaters.
News video from late Monday showed houses submerged to the roof line in raging muddy waters, with people clambering on top. A man, woman and child sat on the roof of their car as waters churned around them.
In Toowoomba, the waters disappeared almost as fast as they arrived, leaving debris strewn throughout downtown and cars piled atop one another.
Queensland officials have said the ultimate cost of the weekslong flooding could be as high as $5 billion.
Explore further: Characterizing permafrost microbes in a changing climate