Australia's third-biggest city Brisbane was besieged Wednesday by once-in-a-century floods that could hit up to 20,000 homes, as the death toll in raging torrents rose to 12.
Thousands of people took refuge with friends and central districts were eerily quiet as the river city of two million battened down for its worst deluge since 1893, as floods spread across vast areas of Australia's northeast.

About 100,000 premises were without power and some residents headed to evacuation centres, as boats and floating restaurants broke their moorings and careered down the swollen Brisbane river, smashing into bridges.

"We are bracing for a one-in-a-hundred-year flood," said state Premier Anna Bligh. "We are bracing for a massive amount of water coming into this river system and it will flood thousands of properties."

Prime Minister Julia Gillard said the magnitude of the threat was "mind-boggling", but urged people in the city, bisected by the winding Brisbane River, to help neighbours deal with what is shaping up as a major emergency.

"If there's someone in your street you're worried about, maybe an older Australian that you haven't seen for a while, maybe give them a knock on the door and make sure they're okay," she added.

Brisbane, the cosmopolitan state capital and economic hub, is the latest and biggest victim of a crisis caused by months of rains that have turned three-quarters of Queensland into a disaster zone twice the size of Texas.

Dozens of suburbs and 2,100 roads are expected to be left under water after the Brisbane river bursts its banks and swamps the city centre, along with other areas, when it peaks at 4:00 am Thursday (1800 GMT Wednesday).

Suncorp stadium was closed after floods and a small explosion and Brisbane's port was open only for emergency supplies as debris sped down the river.

Garbage collections were cancelled and many traffic lights were out of action in several areas.

Isolated ATM cash machines were running out of money and residents were told to conserve drinking water in case supplies are cut. Bligh warned people to find refuge at higher ground and stay there until the danger passed.

"This is a deeply serious natural disaster. Stay in your homes, don't travel unless it's absolutely necessary," she said.

Elsewhere 1,100 people jammed evacuation centres in nearby Ipswich, upstream of Brisbane, where 3,000 homes were deluged. rural condamine was evacuated for the second time, along with two towns in neighbouring state New South Wales.

Some of the inundation was related to flash floods that smashed through towns high in the great dividing range to the city's west on Monday, leaving at least 12 dead as rescuers combing wrecked communities found two more bodies.

State Premier Bligh said the toll was expected to keep rising with "very, very grave fears" for 15 people and 67 listed as missing. The state coroner headed to the stricken Lockyer Valley region, along with grief counsellors.

"This is about bodies caught up in trees, bodies in creeks that we are searching," said disaster coordinator Ian Stewart.

"I'm sure we haven't found the last of the vehicles that have been washed into creeks and things like that. This is a difficult task and it will go on, possibly for days."

Twenty-three people have died in floodwaters in recent weeks, many trying to negotiate fast-running currents by car or on foot. One girl drowned when her foot got stuck between rocks as waters rose.

Prime Minister Gillard said another seven military helicopters had been made available, taking the total number operational in Queensland to 15.

Meanwhile Melbourne, about 1,400 kilometres (850 miles) from Brisbane in Australia's southeast, was on alert for flash floods following incessant downpours also blamed on the disruptive La Nina weather system.