Rescuers are racing to reach people trapped on roofs after a flash flood hurled a tsunami-like wall of water through Queensland, tossing cars like toys, killing at least eight people and leaving 72 missing.
At least four children died and many of those still unaccounted for are families with young children.
The violent surge near the town of Toowoomba after a fresh storm escalated Australia's two-week-old flood crisis and brought the death toll to 18.
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 yesterday. But thunderstorms and more driving rain hampered efforts to send helicopters to help an unknown number of other people still in danger today.
Thousands were being evacuated from flood-prone areas, and residents in some sections of Brisbane -- Australia's third-largest city -- were being urged to move to higher ground as water from Toowoomba's flash flooding worked its way toward the coast.
Queensland state premier Anna Bligh said there were "grave concerns" for at least 11 of the 72 missing.
"This has been a night of extraordinary events," an emotional Ms Bligh said.
"We've seen acts of extreme bravery and courage from our emergency workers.
"We know they're out on the front line desperately trying to begin their search and rescue efforts, and we know we have people stranded and people lost."
She said the death toll stood at eight, but that "we expect that figure to rise and potentially quite dramatically".
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," prime minister Julia Gillard said.
Yesterday's flash flooding struck without warning in Toowoomba, a city of some 90,000 people nestled in mountains 2,300 feet above sea level.
Ms Bligh said an intense deluge fell over a concentrated area, sending a 26ft, fast-moving torrent crashing through Too-woomba and smaller towns further down the valley.
Police Commissioner Bob Atkinson described the events as "an inland instant tsunami, with a massive wall of water that's gone down through the Lockyer Valley".
Today, the water was still pushing its way downstream, flooding river systems as it moved toward the coast.
Thousands were being evacuated from communities in the water's predicted path, and officials closed roads and highways to some areas at high risk of being inundated.
Residents in low-lying regions of the state capital of Brisbane were urged to sandbag their homes and later told to move to higher ground.
"We have a grim and desperate situation," Ms Bligh said. "This took everybody so unawares that there was no opportunity in most cases for people to get to safety."
On the other side of Australia, hot, dry conditions have sparked a wildfire that has destroyed at least four homes.
Around 150 firefighters were battling a blaze about 70 miles south of the Western Australia state capital of Perth today.
There have been no reported injuries.