Floodwaters are expected to surge through 6,500 homes and businesses in Brisbane at a level beyond that seen during the catastrophic 1974 floods.
To the west, the city of Ipswich will also face its worst flooding in almost 40 years.
Premier Anna Bligh says a "frightening" situation is developing in both cities, where evacuations are under way ahead of the Brisbane River's expected peak on Thursday.
She also confirmed there were nine dead from Queensland's flash floods and the toll could get to more than double that.
The premier urged for calm as panicked Brisbane residents fleed the CBD and the rising Brisbane river.
The RNA showgrounds at Bowen hills has been turned into an emergency centre for people who want to self-evacuate.
The deaths of at least nine people were the result of flash flooding at Toowoomba and in the Lockyer Valley, Ms Bligh said.
"We continue to hold very grave fears for the people who are missing," she said, adding that number had been revised down to 66.
"To everyone who has lost a loved one, you are in our thoughts and the thoughts of all Queenslanders and Australians."
She said most of those who remained unaccounted for were from the Murphys Creek, Grantham and Withcott areas, in the Lockyer Valley west of Brisbane.
Ms Bligh said police and search and rescue teams were on the ground at Grantham and Withcott.
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"We do have police on the ground at Murphys Creek but they've been unable to continue with the search and rescue because the weather has significantly worsened," she said.
She said extraordinary scenes had been playing out in the community of Forest Hill, where the entire population of about 300 is being airlifted to safety in Black Hawk helicopters.
Ms Bligh said waters had risen very quickly in the community, between Laidley and Gatton.
She said about half of the population had been flown out to date, with the rest to follow later this afternoon.
"The last report was that 126 people were on the ground still to be airlifted," Ms Bligh said.
Ms Bligh said the situation in Brisbane and Ipswich was very serious.
"Ipswich and Brisbane are now facing their greatest threat and toughest test in more than 35 years," she said, a reference to what's expected to be the worst flooding since both communities were devastated in 1974.
The Brisbane City Council predicted 6500 homes and businesses across 80 suburbs will be inundated over the next few days, and about 16,000 properties will be partially affected by floodwaters.
"We will only pass this test if we are calm, if we are patient with each other ... and if we listen carefully to the instructions we are being given.," Ms Bligh said.
"Now is not a time for panic, now is the time for us to stick together.
"We are facing one of our toughest ever tests, we will only pass this test if we are calm."
Ms Bligh said her advice was that flood levels in the Brisbane River were expected to go beyond the 5.45 metres seen in 1974.
"Current predictions indicate the river will continue to rise into Thursday with flood levels expected to be higher than the 1974 peak," she said.
Ms Bligh said authorities would be holding two-hourly media conferences from this afternoon to keep the public informed of the changing situation.
Deputy Police Commissioner Ian Stewart said a disaster declaration had been made for the lower half of the state to include the greater Brisbane area.
"We would hope that police and emergency workers don't have to use any of those powers," he said.
"As the premier said, this is not the time to baulk when you are asked to leave or asked to support emergency workers in the way we will help prepare the city for this certain flood event."
The powers allow police to forcibly remove people if necessary, Mr Stewart said.
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Mr Stewart said the toll would almost certainly rise.
"I personally believe it will get worse today, and we are waiting on confirmation from a number of concerning reports," Mr Stewart told the ABC, saying the valley communities of Murphys Creek, Withcott and Grantham are of most concern.
"Obviously, our focus now, today, will be to try and help the survivors because we still have a large number of people isolated in the township of Grantham."
Ms Bligh said bad weather was affecting attempts to reach dozens of valley residents who spent the night waiting for help in the wake of a disaster that's stretched emergency authorities to the limit.
"Right now we have every possible available resource deployed into this region to search for those people that we know are missing," she said.
Australian Defence Force resources have been called on in the aftermath of the floods, along with help from interstate.
"This is going to be, I think, a very grim day, particularly for the people in that region, and a desperate hour here in Queensland," Ms Bligh said.
The dead include children, with bodies found in the Toowoomba CBD, at Murphys Creek and Grantham.
More heavy rainfall and possible severe storms are expected in Toowoomba today, which is set to further strain rescue efforts.
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Unstoppable tide hits Brisbane
The Brisbane River has broken its banks with evacuations underway in the heart of the city amid fears Brisbane will see its worst flooding since the 1974 disaster.
Brisbane Lord Mayor Campbell Newman has officially put Brisbane on flood alert and warned residents who live along the Brisbane River and the creeks that they face the potential for "significant flooding".
Cr Newman has warned the estimated number of houses that could be inundated with water is around 6500, while 16,5000 could be partially affected.
He called for calm as workers began fleeing the CBD, clogging up the roads attempting to flee the breaches of the Brisbane river.
Public transport facilities are still running, but many trains and bus services have been canceled due to the flooding.
Office towers on Eagle Street are being emptied, along with businesses in Fortitude Valley, and flood waters are creeping into the inner-city suburb of West End, where residents have been told to get to higher ground.
Evacuations are also underway on Brisbane's northside at Albion and Bowen Hills, the ABC reported.
It's feared Brisbane could be about to experience its worst flood since the catastrophic 1974 event, when at least 6700 homes were partially or totally flooded in the Brisbane metropolitan area.
The Wivenhoe dam was subsequently built to protect the city.
Bureau of Meteorology hydrologist Peter Baddeley said the wall of water seen yesterday in Toowoomba and nearby communities was unlikely to be repeated.
Efforts are underway to reach at least 50 people who took refuge overnight at a school at Grantham, and others at Withcott, where people remain missing and others were forced onto roofs.
More than 40 people were rescued from rooftops by helicopter overnight, police said.
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PM offers whatever it takes to Queensland
Prime Minister Julia Gillard has pledged to provide Queensland with whatever defence force support it needs to cope with the floods, as she warned of more "dark days ahead".
"The nation does need to brace itself for the fact that the death toll as a result of yesterday's flash flooding and walls of water is likely to rise," a sombre prime minister told reporters in Canberra on Tuesday.
Ms Gillard offered her sympathies to the families and friends of those who had lost loved ones.
"To those people who are bereaved, to those who are now waiting for news of their loved ones, I want to say to them as prime minister my thoughts and my sympathies are with you," she said.
"The thoughts and sympathies of all Australians are with you in these dreadful and difficult circumstances."
Ms Gillard said she had been briefed by Australia's national security adviser as well as the head of Emergency Management Australia.
She has also spoken to the commanding officer of Operation Queensland Flood Assist.
"We have helicopters - Blackhawks and Sea Kings - in the air working hand in glove with the Queensland emergency personnel on search and rescue efforts."
Ms Gillard described the circumstances in Queensland as "very dire indeed".
"There are some communities that have been hit by floodwaters for a second time - hit once, evacuated, gone back, and now being hit again."
The government would continue to provide support from the Australian Defence Force.
"And I have made it very clear to Premier Anna Bligh that any resource she needs from the Australian Defence Force will be made available to the people of Queensland to assist them during this very difficult period."
The prime minister plans to visit Brisbane in the next 24 hours, but will avoid inconveniencing search and rescue efforts in Toowoomba and the Lockyer Valley.
She praised the courage and commitment of emergency services personnel who had faced "incredibly harrowing circumstances" during the past 24 hours.
"I thank you for what you have done, what you are continuing to do and what you will do in the days ahead."
Ms Gillard said the cost of rebuilding would be "a very big price tag" but it was too early to tell how big.
"We can only work that out when floodwaters subside ... and we can see what's happened to bridges, roads, airstrips and critical community infrastructure," she said.
Ms Gillard said there were "still more dark days ahead ... but the spirit of Queensland is to face these circumstances with courage and determination."
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Residents witness devastation as evacuations continue
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 said.
A telephone hotline 1300 993 191 has been set up for people seeking information on friends and relatives caught up in the flooding disaster.
Grantham resident Christopher Field told the ABC of the devastation he'd witnessed in the town.
"There were houses floating past ...," he said on Tuesday morning.
He said the force of the water had pushed out the back wall of the old bank on Anzac Avenue, the old post office was collapsing, and the pub had been devastated.
"Grantham shops are being all washed through, cars everywhere in the main street and cars back under the bridge," he said.
Mr Field said the water had receded but was beginning to rise again as more rain fell.
He said some residents were holed up at a local school.
"(There's) a fair few people in there, I had an elderly man come out and just give me a hug earlier, in tears, he'd lost his house."
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