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Queensland premier Anna Bligh says southeast Queensland is facing a bigger flood than the catastrophic deluge of 1974.
Ms Bligh said her advice was that flood levels in the Brisbane River were expected to go beyond the 5.45 metres that nearly drowned the city 37 years ago, killing 14 people.
"Current predictions indicate the river will continue to rise into Thursday with flood levels expected to be higher than the 1974 peak," she said.
"By midnight today river levels at Ipswich will reach the 18-19 metre mark, peaking some time during Wednesday at around 21-22 metres," she said.
"By comparison the 1974 peak was 20.6 metres.
"So in both Ipswich and Brisbane we are anticipating flood levels across the areas of '74 levels and above.
"Ipswich and Brisbane are now facing their greatest threat and toughest test in more than 35 years."
Ms Bligh said authorities would be holding two-hourly media conferences from Tuesday afternoon local time to keep the public informed of the changing situation.
"We are facing one of our toughest ever tests," she said.
"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.
"Now is not a time for panic, now is the time for us to stick together."
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.
Lord Mayor Campbell Newman also warned that the worst of Brisbane's flooding was yet to come, telling the Courier Mail that Thursday was likely to be "devastating".
Cr Newman said latest flood modelling suggested 6500 properties could be inundated, with 16,500 to be only partially affected by floods, the Courier Mail reported.
Workers flee Brisbane CBD
Earlier today, workers fled the Brisbane city centre has the Brisbane Broke its banks in many parts.
Kiwi Brisbane resident Ryan Mudie told nzherald.co.nz cars were streaming out of the city centre as people rushed 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."
There have been reports of people in "panic-mode" rushing to buy supplies and local supermarkets running out of essentials like bread, milk and water.
Devastated Toowoomba braces for more flooding
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."
Death toll rises
The death toll from yesterday's tsunami-like flash floods has risen to nine, with Premier Anna Bligh admitting it could be more than double that in coming days.
Ms Bligh said 66 people were still missing after five were found this afternoon.
While the official toll remains at nine, the Courier Mail reported five more bodies were found in the Lockyer Valley, bringing the toll from yesterday's flash floods in the Toowoomba region to 11.
The dead include a mother and her two children whose car was swept away in the raging floodwaters.
There were reports yesterday of a woman, 42, and her 13-year-old son being swept to their deaths after they were seen clinging to a tree near the intersection of James and Kitchener streets when they were washed away just after 2pm.
A paramedic entered the floodwaters to retrieve the woman's body when it was located under a bridge in Chalk Drive.
The boy's body was located in Kitchener Street.
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