Flood sirens have sounded in Grantham, in the Lockyer Valley Region west of Brisbane, with residents in low-lying areas told to move to higher ground.

Six people sheltered on the roofs of houses in the area on Friday, as emergency services considered helicopter rescues, weather conditions permitting.

Plans are in place to get them to safety "as soon as possible", Queensland Police Deputy Commissioner Steve Gollschewski said.

"My understanding is that they are safe at the moment, they are clear off the water, but it's very important for us to get people in there to assist," he said.

Residents in Gympie, north of the Sunshine Coast, are warned the Mary River could reach levels not realised since major flooding in the area in February 1999.

The river peaked at just under 22 metres during that event 23 years ago, and the Bureau of Meteorology is warning of heavy rainfall over the next 24 hours.

The catchment area recorded falls in excess of 300mm over a 24-hour period.

Emergency services have responded to almost 1000 events in a day, including 41 swift water rescue jobs.

Forty-three schools have been closed across the southeast and the weather event is expected to last until at least Saturday morning.

"This is an incredibly serious and potentially a life threatening situation that's unfolding, and it's escalating quickly in terms of flash flooding, and rapidly rising catchments," Queensland Fire and Emergency Services Commissioner Greg Leach said.

Two people have already died in floods triggered by the weather system, with a 54-year-old man killed trying to ride a motorbike through rising water at Gympie and a 63-year-old woman found dead in a submerged car on the Sunshine Coast.

Emergency services are expecting some properties in the Gympie area to be inundated and are keeping an eye on the Mary River, Mr Gollschewski said.

While Bureau of Meteorology predictions are helpful, emergency services "don't know what's actually going to happen" until the rain falls, he said.

Flood releases from the Wivenhoe Dam are set to begin at 10pm Friday, with warnings several bridges and crossings downstream of the dam are likely to be inundated.

The Department of Transport said flash flooding had cut dozens of roads in Brisbane, Bundaberg, the Darling Downs, the Gold Coast Hinterland, Gympie, Ipswich, Moreton Bay, Noosa, the Sunshine Coast and its Hinterland, and Toowoomba.

Drivers in southeast Queensland are being asked to avoid non-essential travel with road closures across the region, including on the Bruce Highway.

A large trough has been battering the state's southeast - mostly areas north of Brisbane - causing flash floods and moderate flooding for almost four days.

The bureau said an area north of Biggenden recorded 423mm of rain in the 24 hours to 5am on Friday, with 389mm falling in four hours.

Australian Associated Press