Residents of Australia's third-largest city are being urged to leave their homes as deadly flood waters continue to rise along the country's eastern coast.

As of Tuesday morning there had been no forced evacuations in Brisbane, a city of two million and the state capital, but those measures could still come. In the meantime, residents are asked to leave voluntarily.

In total, 20,000 homes and businesses could be underwater by Thursday as the Brisbane River rises, according to Mayor Campbell Newman.

"Today is very significant, tomorrow is bad, and Thursday is going to be devastating for the residents and businesses affected," said Newman.

The warning is largely due to fears that a tsunami-like wall of water is working its way downstream towards the city.

When the water hit overnight in Toowoomba, west of Brisbane, it caused widespread devastation, pushing houses off their foundations and submerging vehicles.

Greg Kowald was driving through the centre of town when the flood hit.

"The water was literally leaping, six or 10 feet into the air, through creeks and over bridges and into parks," the 53-year-old told The Associated Press. "There was nowhere to escape, even if there had been warnings. There was just a sea of water about a kilometre wide."

More than 40 people had to be rescued from rooftops by helicopter, and some 90 people are still missing.

Flooding throughout southeast Queensland has left at least 10 people dead. However, officials have warned the toll could increase dramatically in coming days, due to the possibility many may have drowned in their submerged vehicles and homes.

Relocating residents

While many Brisbane residents heeded the evacuation warning and packed up to flee the city on Tuesday, other residents of low-lying neighbourhoods were desperately setting up sandbags in an attempt to protect their property from the rising river.

"I've never seen rain like this before. The river's been slowly coming up all day," said Tricia Miller, a Brisbane resident. She said people in her neighbourhood were pulling together and helping wherever possible, but there was a mounting sense of tension among many residents.

"We went to the shop earlier and it's pretty cleaned out. There's no bread on the shelves. I think people are a little panicky," she said. "The river's supposed to peak tomorrow and stay high for a couple of days."

State Premier Anna Bligh called on residents of at-risk areas in Brisbane and elsewhere in Queensland to be "overly cautious" and take action to either prepare their property, or get out of town.

"It's better to be inconvenienced and find that your preparations were not necessary, than the alternative," she said. "I think the entire city of Brisbane, the Ipswich region and most of the southeast, needs to prepare ourselves for enormous disruption."

Residents throughout southeast Queensland were being warned to stay off dangerous roads or bridges amid the devastating flooding.

Dramatic images of cars and houses being carried away by a powerful wall of water have been emerging from the region that encompasses Ipswich, Caboolture and Brisbane -- Australia's third-largest city.

A powerful tsunami-like flash flood gushed through Toowoomba in Queensland's Lockyer Valley on Monday, after 36 hours of rain.

The flooding was driven by a powerful freak storm that dropped an estimated 150 millimetres of rain in just 30 minutes, turning a normally small stream into a raging torrent.

It hit with little or no warning.

Death toll could rise

In addition to the 10 already confirmed dead -- five of them children -- authorities have grave concerns for at least 18 of those missing.

In total, 20 people have now been killed in ongoing flooding in Queensland in recent weeks.

So far more than $32 million has been donated to a relief fund set up by the state government.

Prime Minister Julia Gillard warned Australians to brace for more death and destruction as the flood waters continue to rise.

"This is a truly dire set of circumstances for the people of Queensland, with more flooding to come," Gillard told Australian Broadcasting Corp. television. She said she had been "absolutely shocked" by television footage of Monday's flash flood.

The Queensland government website advised residents of affected areas to stay calm and informed, to avoid unnecessary travel and stay off of closed roads, and to cooperate with officials if evacuation becomes necessary.

The entire population of Forest Hill, a small community of 300, was airlifted in military helicopters as floodwaters approached.

In Ipswich, a town of 15,000 between Toowoomba and Brisbane, hundreds moved into evacuation centres or bunked with family members amid warnings that dozens of homes would be swallowed up by the rising Bremer River overnight Tuesday.

With files from The Associated Press