Cities and towns along the central Queensland coast are flooding as the low pressure system that was cyclone Oswald batters the south east corner of the state with cyclonic winds and torrential rain.
Releases from dams that protect the cities of Brisbane and Ipswich are continuing, with falls of up to 300mm expected over the next 24 hours.
Destructive winds gusting to 60 knots are also expected.
Forecasters are expecting conditions "comparable to a high category one cyclone" with wind gusts of 60 knots, or 125kph, in the southeast corner over the next day.
The low pressure system is expected to be over the Sunshine Coast on Sunday afternoon, Brisbane by this evening, and the Gold Coast by late on Sunday night or early on Monday.
It will then move across the border into northern NSW.
The bureau's Queensland weather services manager Richard Wardle says there's a strong chance of more tornadoes like the six that caused severe damage in the Bundaberg region on Saturday and Sunday.
In the state's southeast corner, falls of between 200 and 300mm are expected but that could reach 400mm in some locations, Dr Wardle said.
The central Queensland cities of Gladstone, Bundaberg and Gympie are all experiencing widespread flooding.
There have been dramatic rescues across the region, including in Gympie where the mayor says people had been rescued from rooftops.
There are grave fears for at least one man who remains missing after trying to cross a flooded creek near Gympie on Sunday morning.
Residents of north Bundaberg are being ordered to leave their homes immediately, the ABC reported at midday (AEST), with fears 300 homes will flood and those that don't to be left isolated.
Some residents in south and east Bundaberg also being told to leave, the ABC said.
The Queensland government has asked for army helicopters to be sent in to help, the ABC reported.
And Premier Campbell Newman has called on Prime Minister Julia Gillard to say the state will need help to deal with a crisis spanning many communities simultaneously.
Mr Newman said it was highly likely the army would be called on for more support, particularly in the clean-up phase.
"I know she and her government will provide the support we need," Mr Newman told reporters in the tornado hit community of Bargara near Bundaberg.
"The challenge now is that we've got multiple events going on."
In the central Queensland city of Bundaberg, residents are braced for the Burnett River to peak at levels higher than in 2010/11, when floods caused significant damage to the city.
Up to 200 homes and 100 businesses are expected to flood, and evacuations are being ordered in north Bundaberg, with some residents in the south and east also being told to head to evacuation centres.
At Gympie, south of Bundaberg, residents have been rescued from rooftops, Mayor Ron Dyne told AAP.
And the search is continuing for a 27-year-old man who was swept away while trying to cross the Widgee Creek near Gympie. His two companions were rescued.
The Mary River is expected to peak at 17 metres - just shy of levels reached during floods that devastated the region in 2011.
The river is so swollen that it looks more than a kilometre wide, Gympie local Suzanne Chapman told AAP.
"We're not far off from starting to evacuate," said Ms Chapman, who works at the Royal Hotel in Gympie.
"The sports bar will get a metre or so of water if the river stops at 17 metres but if it goes higher the restaurant will go under. The power hasn't been cut yet but we've been told it might be soon."
At Gladstone, floodwaters appear to be dropping but 900 homes have been evacuated in low lying areas.
And there are reports of homes flooded to their ceilings at Baffle Creek, between Gladstone and Bundaberg.
The Insurance Council of Australia has declared a Queensland-wide catastrophe.
Severe weather is now affecting the Sunshine Coast, and the Moreton Bay Regional Council areas north of Brisbane, causing severe beach erosion and three to four metre swells.
Brisbane city is also being lashed by cyclonic winds gusting above 120kph, and the city's disaster co-ordination centre has been activated.
There are already reports of localised flooding in Brisbane, including in Bayside suburbs.
