Deadly floodwaters that have cut a swath across northeastern Australia flowed onto the streets of the nation’s third largest city on Wednesday, forcing people to flee suburbs and skyscrapers as rescuers elsewhere searched for 67 people still missing.

Almost 20,000 homes in Brisbane were expected to be swamped in the city of about two million by the time the Brisbane River reaches its expected peak on Thursday, Mayor Campbell Newman said. The figures were constantly being revised as the threat became clearer - and it was getting consistently worse.

At least 22 people have died in Australia’s northeastern state of Queensland since drenching rains that began in November sent swollen rivers spilling over their banks, inundating an area larger than France and Germany combined.

The crisis escalated when a violent storm sent a 26-foot (eight meter), fast-moving torrent - described as an “inland instant tusnami” - crashing through the city of Toowoomba and smaller towns to the west of Brisbane on Monday. Twelve people were killed in that flash flood, and 67 remain missing.

Emergency sirens wailed throughout Brisbane on Wednesday afternoon. 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. Some streets and riverside parks were covered with water, though no major flooding was reported early Wednesday.

Two evacuation centres have been established in the city and Newman said up to 6,500 were expected to use them in coming days. Officials have urged anyone in a growing list of low-lying suburbs to prepare their homes, then get out to stay with friends and family and keep off the streets.

“This incident is not a tourist event - this is a deeply serious natural disaster,” Queensland state Premier Anna Bligh said. “Stay in your homes. Do not travel unless it is absolutely necessary.”

The Brisbane River broke its banks on Tuesday and was continuing its rise on 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, Ms. Bligh said.

Some residents waited in line for up to four hours on Tuesday to get sandbags being handed out at emergency services depots. Supermarket shelves were emptied of bottled water, milk and batteries.

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, Ms. Bligh said.

“I know that this is going to be very difficult for people,” Ms. 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 on Tuesday 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. His wife and four children left to stay with relatives after police came and asked everyone on their street to evacuate.

Marchant and two neighbours 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 (neighbour’s) pontoons just floated off,” he said on Wednesday. “It was amazing.”

For weeks, the flooding had been a slow-motion disaster, devastating wide swaths of farmland and small towns and virtually shutting down the state’s vital coal industry. 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 on Tuesday and helicopters and other emergency vehicles were moving into the worst-hit parts of the valley on Wednesday. Ms. Bligh warned that the death toll would likely rise as rescue officials gained access to the devastated areas.

Relentless rains that have been pounding the region cleared on Wednesday, but Ms. Bligh said while the break in the weather would help rescue officials, it would have no impact on the flood threat to Brisbane.

“We can take no comfort from that blue sky,” Ms. Bligh said. “The water and the rain have already done their damage - they are in the catchment, and they are on their way down the river system.”

The city of Ipswich, home to about 15,000 people, was swamped on 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 centres, Mayor Paul Pisasale said. Water levels there were expected to hit their peak later on Wednesday.

In Brisbane, the river is expected to crest on Thursday at levels higher than those of a devastating 1974 flood, though the damage is expected to be less because that flood struck with little warning. The mayor urged people to stay away from downtown, and most office towers stood virtually empty on Wednesday.

“This is a truly dire set of circumstances,” Prime Minister Julia Gillard said.

Steph Stewardson, a graphic designer, said there was an exodus from a downtown area 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.

Ms. Bligh said last week the cost of the floods could be as high as $5 billion, the latest figure available.

The floods have also reached the bordering state of New South Wales, with about 4,500 people stranded, though the situation was not yet as dire as in Queensland. New South Wales is Australia’s most populated state and contains its biggest city, Sydney.

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