Brisbane and Ipswich are on full flood alert with two major rivers expected to hit record levels.
The Brisbane River is set to rise to 4.5 metres at about 2.30pm (AEST) on Wednesday and could go higher than the 1974 floods that reached 5.45 metres.
The Bremer River at Ipswich is expected to reach 22 metres on Wednesday morning - 1.5 metres above the 1974 levels. A third of the city is expected to go under water.
Conditions are described as similar to when Brisbane was hit by fatal floods on Australia Day weekend after Cyclone Wanda in 1974. Fourteen people died and almost 6000 houses were flooded.
Brisbane is Australia's third largest city, with a population of two million people. Streets are largely empty.
A steady stream of debris is floating down the Brisbane River, including boats ripped from their moorings by the force of the current.
The city is facing a combined surge of water from the flooded Lockyer Valley and the Wivenhoe Dam, which is so full that controlled discharges are being carried out.
Latest flood modelling suggests 9000 houses in Brisbane will be significantly affected by flooding.
Another 30,000 properties are expected to experience some impact.
Disaster declarations have been issued for the Brisbane, Bundaberg, Dalby, Gladstone, Gold Coast, Gympie, Ipswich, Logan, Maryborough, Rockhampton, Roma, Sunshine Coast, Toowoomba, Warwick and Redcliffe districts.
Power has been shut off to Ipswich and was also cut to more than 100,000 customers in Brisbane city from 7am onwards (AEST) on Wednesday.
The BBC reports the forecast is for more rain to come.
Supermarket shelves have been stripped bare. An ABC correspondent told Summer Report that milk and bread can no longer be found anywhere.
Darkest hour - State Premier
Three quarters of Queensland is now a disaster zone as the flooding takes hold in more areas.
Earlier, flash floods in the city of Toowoomba, west of Brisbane, killed at least 10 people on Monday. Ninety more people are missing. The ABC reports serious concern is held for 18 of them.
Prime Minister Julia Gillard has warned that the death toll is likely to rise.
She is now in Brisbane and has pledged to provide Queensland with whatever support it needs from the Defence Force to cope with the floods.
State Premier Anna Bligh has called the floods Queensland's darkest hour since they began at the end of November.
She called the event a complete freak of nature, saying the flooding has come out of nowhere.
We are facing one of our toughest ever tests, we will only pass this test if we are calm, she said.
Now is not a time for panic, it is a time for us to stick together.