Brisbane braces as flood hits peak (VIDEO)

Brisbane braced for the worst flooding yet seen in Australia as the river was expected to peak over 17 feet around 4 a.m. Thursday morning (1 p.m. ET Wednesday).

After a day of calm, sunny weather, during which the normally sedate Brisbane River turned into a raging torrent that broke its banks and flooded the downtown area, residents are being told to prepare for a "shocking and remarkable challenge."

Thousands of homes and businesses across the city of two million have been submerged up to the second story. The entire downtown has been closed off and the city has been virtually cut in half, according to reports.

Brisbane is the latest city to be affected by the devastating floods, which have inundated a large swath of Australia's northeastern Queensland state. A map of the flood-region shows an area the size of France and Germany combined.

The most recent estimates suggested that 20,000 homes, 3,500 businesses and 2,100 roads would be hit by the massive flood when the swollen Brisbane River peaked.

The state premier, Anna Bligh, told reporters Wednesday that the death toll from the flooding had risen to 12, but that a further 67 people had been reported missing.

"We are preparing for the worst natural disaster in our history," Bligh said.

Deputy Police Commissioner Ian Stewart told The Sydney Morning Herald that recovery teams were searching buildings "completely destroyed" by floodwaters, and combing trees, creeks and cars for bodies.

"Our hope is that we actually might find some survivors but the chances of that are quite remote," he said.

Brisbane Mayor Campbell Newman urged residents in affected areas to evacuate before the morning.

"It's already wreaking devastation," he said, the Australian Broadcasting Corporation reported. "Lots of flooded homes and businesses across Brisbane, but it'll go another meter higher in the early hours of Thursday morning."

Newman said sewage treatment plants might be affected, contaminating floodwaters.

"Another reason to stay out of the water, don't cop a mouthful of it, have a good decent shower and clean up afterwards," he said.