A massive clean-up for flood-devastated Brisbane may take months while reconstructing the city fully could take years, authorities warned on Thursday.

Almost 12,000 homes and 2500 businesses remain inundated amid rising concerns about "live" electricity and the city has all but shut down.

Almost 150 roads are partially or fully flooded.

Overnight, more than 1000 residents took refuge at evacuation centers, with St Lucia, West End, Rocklea and Graceville among the worst affected.

Lord Mayor Campbell Newman said the council will throw every resource at returning Brisbane to normal.

"The clean-up effort, ultimately, will take many, many months," he told reporters on the banks of the Brisbane River.

"The reconstruction effort on people's homes is going to take much longer than that - that will take a year and a half to two years."

The first priority will be clearing roads, particularly major arterials such as Coronation Drive and the Ipswich Motorway.

Helicopters will begin a surveillance scan of Brisbane on Thursday to assess where to begin, while volunteers, particularly tradesmen, are being urged to lend a hand to get homes back in order.

The river is due to peak again on Thursday afternoon, but at lower level of about 4.2 meters.