The surging, muddy waters reached the tops of traffic lights in some parts of Brisbane, and the city's mayor said at least 20,000 homes were in danger of being inundated.
At least 22 people have died and more than 40 are missing across Australia's northeastern state of Queensland since drenching rains that began in November sent swollen rivers spilling over their banks, flooding an area larger than France and Germany combined. Brisbane, the state capital with a population of 2 million, is the latest city to face down the waters, and officials expect the death toll to rise.
On Wednesday, Brisbane residents who had spent two days preparing took cover on higher ground while others scrambled to move their prized possessions to the top floors of their homes. Some stacked furniture on their roofs.
The Brisbane River is expected to reach its highest point on Thursday. After days of bad news in which figures were constantly being revised, the Bureau of Meteorology late Wednesday delivered a small and rare positive forecast - the floodwaters would crest about a foot (30 centimeters) lower than earlier thought.
If correct, the new forecast meant the waters would not reach the depth of 1974 floods that swept the city. Queensland Premier Anna Bligh said the news was welcome, but of little comfort.
"This is still a major event, the city is much bigger, much more populated and has many parts under flood that didn't even exist in 1974," she said. "We are still looking at an event which will cripple parts of our city."
The dragged-out crisis escalated when a violent storm sent a 26-foot (eight meter), fast-moving torrent - described as an "inland instant tsunami" - crashing through the city of Toowoomba and smaller towns to the west of Brisbane on Monday. Twelve people were killed in that flash flood. Late Wednesday, Bligh said the number of missing had been revised down to 43.
"This is a truly dire set of circumstances," Prime Minister Julia Gillard said.
The Brisbane River broke its banks on Tuesday and was continuing its rise Wednesday - partly controlled by a huge dam upstream that has had its floodgates opened because it is brimming after weeks of rain across the state.
Water levels were expected to stay at peak levels until at least Saturday, but many people won't be able to access their homes for several days beyond that, Bligh said.
The flooding has transfixed Australia and is shaping up to become the nation's most expensive disaster, with an estimated price tag of at least $5 billion. The relentless waters have shut down Queensland state's crucial coal industry and ruined crops across vast swaths of farmland.
Brisbane's office buildings stood empty Wednesday with the normally bustling central business district transformed into a watery ghost town. Most roads around the city were closed, and people moved about in kayaks, rowboats and even on surfboards. One of the city's sports stadiums, which hosts international rugby games, was flooded with muddy, chest-deep water.
Boats torn from their moorings floated down the rising river along with massive amounts of debris. A popular waterside restaurant's pontoon was swept away by the current and floated downstream. Officials said they would probably have to sink a barge that serves as an entertainment venue, to stop it from breaking free and becoming a floating torpedo.
Officials opened three more evacuation centers on Wednesday, and Newman said there was now room for 16,000 people to take shelter. Officials have urged people to get to higher ground and keep off the streets unless absolutely necessary.
Energex, the city's main power company, said it would switch off electricity to some parts of the city starting Wednesday as a precaution against electrocution. Almost 70,000 homes were without power across Queensland by Wednesday afternoon, Bligh said.
"I know that this is going to be very difficult for people," Bligh said. "Can I just stress: Electricity and water do not mix. We would have catastrophic situations if we didn't shut down power."
Darren Marchant spent all day moving furniture and other household goods to the top floor of his home, near the river in the low-lying Brisbane suburb of Yeronga, which is expected to be inundated. He and two neighbors watched in awe as dozens of expensive boats and pontoons drifted past.
"We were watching all kinds of debris floating down the river - one of the (neighbor's) pontoons just floated off," he said Wednesday. "It was amazing."
For weeks, the flooding had been a slow-motion disaster, devastating wide swaths of farmland and small towns. On Monday, the crisis took a sudden, violent turn, with a cloudburst sending a raging torrent down the Lockyer Valley west of Brisbane. Houses were washed from their foundations and cars tossed about like bath toys in what Police Commissioner Bob Atkinson described as "an inland instant tsunami."
Hundreds had to be rescued by helicopter Tuesday and emergency vehicles were moving into the worst-hit parts of the valley on Wednesday. Bligh warned that the death toll would likely rise as rescue officials gained access to the devastated areas.
In the Lockyer Valley town of Grantham, entire houses that had been swept off their foundations sat in sodden heaps of jumbled debris. Waters that had submerged a railway bridge receded, exposing an avalanche of twisted wreckage caught in its foundation: furniture, a "for sale" sign, a child's swing set, even a dead cow.
The city of Ipswich, home to about 15,000 people, was swamped Wednesday by the water heading Brisbane's way. By the afternoon, 3,000 properties had been inundated, and 1,100 people had fled to evacuation centers, Mayor Paul Pisasale said. Video from the scene showed horses swimming through the brown waters, pausing to rest their heads on the roof of a house - the only dry spot they could reach.
Steph Stewardson, a graphic designer, said there was an exodus from Brisbane's downtown around lunchtime Tuesday with people streaming out of skyscrapers as the river broke its banks. Stewardson, 40, hopped in her car and crossed the swollen river to collect her dog, Boo, from daycare while waters started covering the boardwalk stretching along its banks.
Stewardson took shelter in her house and plans to stay there - for now.
"I'm about 800 meters (half a mile) from the river on a hill, so I think it's going to be OK," she told The Associated Press.
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Gelineau reported from Sydney.
