A second body has been found in the Lockyer Valley, west of Brisbane, lifting Queensland's flood toll to six.

The body of a 34-year-old man was found in a car submerged in Sandy Creek at Glen Cairn, near Gatton, on Wednesday morning.

Police have confirmed a second body was found nearby.

The 34-year-old was travelling to work from Gatton to Mulgowie at on Sunday morning with his 25-year-old mate following in a second car.

Both were swept off the road by rising floodwaters.

Police divers discovered the second man's body on this afternoon.

Four other people have died in the disaster, all in Queensland, including a three-year-old boy who was hit by a falling gum tree while watching floodwaters in Brisbane. His pregnant mother remained in a critical condition in hospital.

The devastation caused by Cyclone Oswald across Queensland and neighbouring New South Wales was expected to linger for days, with thousands of homes inundated and entire communities isolated.

In one of the worst hit areas, Bundaberg, teams trained to find earthquake victims were to sweep flood-bound homes amid fears elderly or disabled people could be trapped.

The river that runs through the Queensland sugar city broke its banks during the storm, sending fast-moving, muddy water pouring into streets and homes.

Thousands of residents fled to evacuation centres, while patients at the local hospital were airlifted to Brisbane as a precaution. There were fears they would be stranded in the shelters for days.

Four helicopters and several civilian choppers also performed more than 1000 air rescues, lifting stranded residents off roofs in North Bundaberg.

The four urban search-and-rescue teams would soon be deployed to the city, where 2000 homes and 200 businesses have been inundated, leaving 7500 residents displaced. 

FEARS OF RUNNING DRY

Some suburbs in Brisbane could run out of water today, city authorities have warned.

Flooding in the Brisbane River catchment has caused major  problems at a water treatment plant at Mt Crosby.

It's partially back on line, but full restoration won't happen for another day and a half and reservoirs that service some  southside suburbs are running very low, Premier Campbell Newman said.

Supplies of bottled water had been positioned on the southside  in case supplies ran out, Newman said.

''If any reservoir runs dry, we'll be immediately providing that  for distribution to local areas,'' he told ABC television.''This is very serious.''

LUCKY ESCAPE

A tense night for residents in the northern NSW town of Maclean ended in relief after the swollen Clarence River peaked below the expected level.

The State Emergency Service said there were no reports of inundated properties and the river was holding steady at 3.08 metres, below the predicted 3.2-metre peak.

However authorities were closely monitoring conditions as the levee wall in Maclean is 3.3 metres.

More than 750 residents in low-lying parts of the town were ordered to evacuate last night based on the Bureau of Meteorology's predicted flood peak this morning.

The SES said the evacuation order remained in place.

Floodwaters have been receding today throughout both stated but dozens of communities remain cut off and partly underwater.

-  Sydney Morning Herald, The Brisbane Times and AAP