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Residents in several parts of Brisbane, including the CBD, are being urged to flee to higher ground as the Brisbane River breaks its banks and the flooding that has devastated large areas of Queensland reaches Australia's third largest city.

The Courier Mail reports a wall of flood waters is heading towards Brisbane, with residents warned to prepare for a major flood and parts of the CBD being evacuated.

There have been no forced evacuations in Brisbane, although the flood situation is rapidly worsening.

According to a Queensland Police Service alert, inner-city dining precinct Eagle St is now underwater at the pier and businesses are being evacuated.

The Courier Mail also reports five more bodies have been found in the Lockyer Valley, bringing the toll from yesterday's flash floods in the Toowoomba region to 11.

Kiwi Brisbane resident Ryan Mudie says cars are streaming out of the city centre as people rush to escape rising flood waters.

He took 40 minutes to complete his usual seven minute journey home after being evacuated from his office earlier today.

Those who can't escape the city are booking hotel rooms and waiting out the storm, he says.

"There are people that won't be with their families tonight. The CBD has been closed down. It's just a bit of a shambles."

Brisbane River has burst its banks in places and heavy rain is continuing to fall, says Mr Mudie.

Authorities are telling residents the flood will only peak at about midday tomorrow, he says.

"It's just pissing down round here. It's not stopping. And it's not going to stop today."

Police say there have been no forced evacuations in Brisbane but the flooding situation is rapidly worsening.

"We urge people to plan their journey home carefully, and to leave plenty of time to get home before water cuts off a number roads in low-lying areas," says a message on Queensland Police's Facebook page.

"Traffic is building as people do attempt to get home and ongoing rain is slowing journeys. All motorists are urged to drive to conditions, take care on the road and avoid driving into floodwater."

In Toowoomba, communities are bracing for more flooding as heavy rain continues to fall.

Resident Ange Symonds, 25, says authorities say there will be 200ml more rain in the area in the next two hours.

"They've evacuated everyone from the CBD. They're still going to try with the clean up but if it starts to flood again then there's not much they can do except get out."

She was on the outskirts of Toowoomba when the flood hit last night and returned this morning to find the town she grew up in all but unrecognisable.

Flooding has destroyed a flour mill which employed 300 people, she says.

"It's an absolute mess. It's like a ghost town really. I've never in 22 years seen anything like this. I've never seen so much rain in my life."

Ms Symonds and others from her church congregation spent today handing out food to emergency workers.

"They've been working non-stop. They spent the morning searching through the backseats of cars for bodies."

'Whole families' unaccounted for

Whole families are unaccounted for after flash floods in southern Queensland, with eight confirmed dead and Premier Anna Bligh warning the toll could rise dramatically.

Ms Bligh said authorities were considering declaring a mega-disaster zone over a "significant region" of Queensland, a move that would be unprecedented in the state's history.

"We are just looking for an appropriate northern line and everything south of that would be declared," she said.

The premier said four of the confirmed deaths were children, and there were fears for many other people, with 72 unaccounted for.

"We have eight confirmed deaths at this point but we expect that figure to rise and potentially quite dramatically," Ms Bligh told reporters today.

Ms Bligh said "whole families" were unaccounted for after flash flooding hit the city of Toowoomba west of Brisbane, and the Lockyer Valley at the foot of the Toowoomba range.

"We're hoping and praying that many of those 72 people have gone to safety overnight and will be found safe and well," she said.

"But given the circumstances we hold very grave concerns for many of those people.

"... many of the people who are stranded or unaccounted for are families and young children and some of those who have lost their lives overnight are young children, including a mother and two children in a vehicle."

Ms Bligh said bad weather was hampering search efforts where people were still stranded in "dire and critical circumstances".

Specialist teams have been deployed to the region and rescue teams are on standby, as well as additional fire and rescue officers.

But weather is stopping them from being deployed, she said.

"We have a grim and desperate situation," the premier said.

"... this took everybody so unawares that there was no opportunity in most cases for people to get to safety."

Until emergency workers could reach affected areas, it would remain unclear if those forced to take shelter on roof tops in the valley had reached safety.

Evacuations continued across the Lockyer Valley on Tuesday, amid more warnings of flash flooding.

Deputy Police Commissioner Ian Stewart said there were grave concerns for the communities of Murphys Creek and Grantham.

"It is expected we will find further persons deceased in those areas. The number we just don't know at this stage," he told reporters.

Ms Bligh said the flooding west of Brisbane would have major implications for the Wivenhoe Dam, which releases water into the Brisbane River.

She said those releases were not optional and must continue to ensure the dam functions properly.

On Tuesday, floodwaters began to creep into West End in the heart of Brisbane, with police saying people who live or are near the Brisbane River should move to higher ground.

There had been no order as yet for mandatory evacuations in the suburb, police said.

Brisbane is expected to see further flooding overnight and into Wednesday, with the council warning about 200 properties could be flooded, most in the areas of Rocklea, Albion, Milton and Auchenflower.

Ms Bligh said modelling was being carried out to determine how the Toowoomba deluge would affect the situation in the capital.

Ms Bligh said it was clear the state was now mired in a very different sort of disaster.

"It is testing our emergency resources and it will test us as a community and as people," she said.

"It might be braking our hearts at the moment, but it will not break our will."

Four defence force helicopters have been sent to the flood zone along with extra swift-water rescue experts, Emergency Management Queensland told AAP.

More than 50 people remain holed up at a school in Grantham - a town that looks like a cyclone has swept through it, with houses and businesses destroyed and cars wedged in trees.

Meanwhile, the flood crisis continues to play out in other parts of the state.

Overnight, residents in Dalby and Chinchilla were evacuated for the second time in a fortnight.

There was also flooding in Gympie with the water moving down the Mary River towards Maryborough.

Ms Bligh and Mr Stewart repeated warnings for people to stay out of floodwaters.

"This water is deadly and it's not to be played with," the premier said.

View Flooding devastates Queensland in a larger map