The death toll looks set to rise while a desperate search for survivors begins in the wake of devastating flash floods across southern Queensland yesterday.

Eight people are dead and 72 missing after flash floods hit the Darling Downs city of Toowoomba and several smaller townships in the Lockyer Valley.

Among the fatalities, four people died in Toowoomba, two women and a child died at Grantham and nearby at Withcott police say nine people are missing.

At least 50 people have sought refuge in a primary school at Grantham.

Police say more than 40 people were rescued from rooftops by helicopter overnight.

The flood threat is today expected to extend to the state's capital, Brisbane.

The Defence Force is joining the search for scores of residents missing in the Lockyer Valley, west of Brisbane.

The area was devastated yesterday afternoon when flash floods that have been likened to a tsunami swept through the region.

Premier Anna Bligh says a major incident room has been set up in Brisbane to coordinate a rescue operation.

"All possible resources, including ADF resources, will be activated at first light tomorrow to begin further search and rescue," she said.

"We will be getting out at first light to continue the search for those who have been notified as missing and to continue the rescue efforts for those who have been stranded on rooftops.

"We've certainly had some unconfirmed reports of further missing people in the Grantham area."

Ms Bligh says the latest developments have stretched emergency authorities.

"We've already called on helicopters from northern New South Wales and they are stationed here ready to take off at first light," she said.

"The scale of what is unfolding in the Lockyer Valley and again in the south-west will test our resources and we have plenty of offers of help from other states and we'll certainly call on those as we need them."

She described the flash flooding at Toowoomba as a freak of nature.

"Mother nature has unleashed something shocking out of the Toowoomba region," she said.

"What we have here in Queensland is a very grim and desperate situation."

Federal MP Ian Macfarlane says he spoke to the Prime Minister last night about the devastation in Toowoomba and the Lockyer Valley.

"Buildings that have been there for over 100 years had water into them and then burst windows," he said.

"All floor stock has been swept down the river. Individual small businesses will have been devastated. There'll be structural issues with the buildings themselves. Bridges and roads have been torn up."

Community in shock

Residents in the Lockyer Valley are still reeling after yesterday's devastating flood.

Mayor Steve Jones says everyone is in shock.

"I think they're all a bit amazed at what's happened and it's really taken them by surprise, but at the end of the day they've got enormous ability to stick together," he said.

"They've been through many bushfires and that sort of thing and the community spirit is fantastic and they'll make it happen."

Police say the Warrego Highway is closed indefinitely at the Toowoomba range and at Glenore Grove in the Lockyer Valley.

The highway is also closed at the Gatton bypass.

Further west at Chinchilla the river is expected to reach seven metres and an evacuation centre has been set up.

The weather bureau's Brett Harrison says severe weather conditions are set to continue today.

He says more rain is forecast for the Darling Downs and further north in the Sunshine Coast hinterland.

"Everything is so saturated at the moment that it doesn't take too much rain to increase the water levels on the ground at the moment, so it's going to continue until we see the upper low weakening and moving away," he said.

Threat spreads

The flood threat is expected to extend to the state's capital today.

The Brisbane City Council says more than 200 homes and businesses along the river are at risk.

Most of those properties are at Rocklea, Albion, Milton and Auchenflower.

Lord Mayor Campbell Newman says the flood threat is real and that residents should heed the warning.

"We are giving you a scenario at the moment that people can actually plan on and make preparations on," he said.

"We hope it doesn't happen, but if it does and we didn't tell people, then they'd quite rightfully be upset."

Ipswich Mayor Paul Pisasale says he does not expect the city's CBD will be flooded when the Bremer River peaks at noon.

But he says several homes at Karalee could be swamped.