Police are urging people to evacuate parts of Australia's third largest city - Brisbane - as huge floods approach the Queensland state capital.
The city's mayor has warned that 6,500 homes and businesses are set to flood.
The waters are rising fast; one local official said he saw the river level go up by 1.5m (4ft 10in) in just an hour.
The state is suffering its worst floods in decades. In Toowoomba, just west of Brisbane, flash floods killed at least nine people with at least 70 missing.
A raging torrent of water hit Toowoomba on Monday without warning, following more than 36 hours of incessant rain.
A huge rescue operation was mounted as many residents clinging to trees or railings for their lives, or trapped in cars or on the roofs of buildings.
At least two of the dead were children, and Prime Minister Julia Gillard has warned that the death toll is likely to rise.
State Premier Anna Bligh called the flash floods Queensland's "darkest hour" since the floods began at the end of November.
Some 200,000 people have been affected across the state by the floods, which have caused billions of dollars worth of damage.
'Bravery and courage'
In Brisbane, sandbags have been given out to residents as the flooding is expected to peak Wednesday.
Mayor Campbell Newman warned 6,500 homes, businesses and other properties were likely to be flooded by Thursday.
"Today is very significant, tomorrow is bad, and Thursday is going to be devastating for the residents and businesses affected," he said.
One Brisbane resident told the BBC that supermarkets were already running out of food.
"I now live in Indooroopilly, a suburb in Brisbane and unfortunately now it has been listed as high-risk area," said Jiao Yu.
"I went to the supermarket just now and almost all the food has been taken - all people I saw on the streets seemed to be anxious, and shops and railways here have begun to stop running."
Further upstream in Toowoomba, a massive search and rescue operation is continuing after deadly flash floods, with helicopters winching people to safety.
"This has been a night of extraordinary events," Ms Bligh told a news conference on Tuesday.
"We've seen acts of extreme bravery and courage from our emergency workers. We know they're out on the frontline desperately trying to begin their search and rescue efforts, and we know we have people stranded and people lost," she said.
She called the event "a complete freak of nature", saying the flooding had come "out of nowhere".
Toowoomba resident Charlie Green told the BBC he was stranded by the floods.
"It would be ironic if it wasn't so tragic," he said. "Toowoomba sits in the cradle of an extinct volcano about 2,000 feet (610m) above sea level, and we have just endured 10 years of drought, unable even to wash our cars with town water for the last several years.
"We are going to sit tight until we're sure that it's safe to move around. The flooded creeks are within a mile of our house so we can't get anywhere.
"We can't even get down the hill. We'll be stocking up on supplies from local shops."
The flooding has been so widespread that while some communities are still bracing themselves for the worst, in others the clean-up is well under way.
The forecast is for more rain to come, and there are reports of flooding in neighbouring New South Wales.
Prime Minister Julia Gillard has warned that the recovery will take a long time.
