The Brisbane River has broken its banks with evacuations underway in Brisbane, the Australian third largest city, amid fears it will see its worst flooding since the 1974 disaster, local media reported on Tuesday.

Office towers are being emptied, along with businesses in Fortitude Valley, a suburb of central Brisbane, and flood waters are creeping into the inner-city suburb of West End in South Brisbane, where residents have been told to get to higher ground.

Evacuations are also underway on Brisbane's northside at Albion and Bowen Hills, the Australian Broadcasting Corporation (ABC) reported.

Brisbane Lord Mayor Campbell Newman is due to brief the media on Tuesday afternoon.

It's feared Brisbane may experience its worst flood since the 1974 catastrophe, which at least 6,700 homes were partially or totally flooded in the Brisbane metropolitan area.

The Wivenhoe dam was subsequently built to the protect city's west.

The Brisbane City Council has released a list of 32 at-risk suburbs.

On Monday it warned 200 properties could be flooded, most in the areas of Rocklea , Albion, Milton, and Auchenflower in Brisbane.

But the situation has worsened since then, and that warning was issued before flash flooding tore through Toowoomba and the Lockyer Valley west of Brisbane.

Some residents at Ipswich were also evacuating areas close to the rising Bremer River on Tuesday.

The floods of 1974 flooded 1,800 homes and businesses in Ipswich, and 41 homes were washed away.

On Tuesday, the Wivenhoe Dam was at 173 percent capacity. It can take up to 200 percent. Somerset dam, which feeds into Wivenhoe, was at 160 percent, the South East Queensland (SEQ ) Water Grid said.