Central Brisbane is a ghost town, with electricity cut and thousands urged to either evacuate or stay at home.
West of Brisbane, the city of Ipswich is being swamped by flood waters in a situation described as "total chaos".
The death toll from the flash floods in Queensland is 12 so far, with many others reported missing.
State Premier Anna Bligh told Australian television that the number of missing west of Brisbane in the Lockyer Valley had risen.
"The police now are searching for more than 90 people," she said. "These are people who their families can no longer contact or find."
She added: "The one good news is the rain has cleared, so we've now got a number of search and rescue teams all ready to deploy into the valley to really start what might be a very gruesome search this morning for bodies and our police and emergency workers there are going to have a very tough day.
"I think we will all be shocked by what they will find."
'Serious event' The central business district in Brisbane was almost totally deserted on Wednesday, hit by a power cut that was intended to prevent generators becoming a fire risk if flooded.
An inundated sports ground in the severely-hit Ipswich
Shops put up signs that they would not open.
The
Brisbane Courier Mail said 50 of the city's suburbs would be hit by flooding during the day, and it quoted Mayor Campbell Newman as saying some areas would be completely submerged.
He said that Thursday would "be devastating for the residents and businesses affected".
The Brisbane river had burst its banks at Yeerongpilly and Indooroopilly, flooding streets. The paper quoted city council flood modelling as predicting that 40,000 properties would be affected.
More than 6,500 Brisbane residents are expected to take refuge in three evacuation centres.
Local media said the peak for Brisbane could be in the early hours of Thursday and the level would remain until Saturday. It is predicted to be higher than the 5.45m reached in the devastating 1974 floods.
Continue reading the main story Brisbane's Major Floods
- 1841 - 8.4m
- 1844 - 7m
- 1864 - 3.8m
- 1887 - 3.8m
- 1889 - 3.8m
- 1890 - 5.3m
- 1893 - 8.3m
- 1907 - 3.5m
- 1974 - 5.45m
Ms Bligh said: "I want to reassure particularly the people of Ipswich and Brisbane that we have every available resource deployed to assist them over the next couple of days in what we expect to be a very serious event."
Brisbane is facing a combined surge of water from the flooded Lockyer Valley and the Wivenhoe Dam, which is so full it has been forced into controlled releases.
High, or king, tides on Thursday will exacerbate the problem.
Sandbags have been given out to residents in the city of two million people.
More rain The
Queensland Times said 3,000 homes were under water and 1,100 people have gone to evacuation centres in the city of Ipswich.
It said the Bremer river in the city was now expected to rise to around 20.5m on Wednesday afternoon, higher than the 1974 peak.
The paper quoted councillor Paul Tully as saying the water was rising by one metre an hour. It was "total chaos", he said. Train services have been suspended to the city.
Toowoomba resident Jenny Mullholland on the flood aftermath
Ipswich Mayor Paul Pisasale said he expected flood levels to drop within the next 36 hours, allowing the clean-up to begin after.
Queensland's flooding has caused billions of dollars worth of damage and affected 200,000 people.
The most deadly floods so far hit Toowoomba, just west of Brisbane, on Monday.
Toowoomba mayor Peter Taylor told BBC Radio 4: "We're working 24 hours a day responding on a emergency number for people who need any assistance in terms of evacuation."
The forecast is for more rain to come for some areas, and there are reports of flooding in neighbouring New South Wales, with the Clarence River expected to peak at 7m.
