Thousands of people were forced to evacuate their properties as the Brisbane River hit the first of two predicted major tidal flood peaks in the Queensland capital this afternoon.

Homes and businesses were inundated in more than 30 suburbs as the river, swollen by water being discharged from the overloaded Wivenhoe Dam, reached 4.2 metres and rising.

The surging waters are expected to reach around 5.5 metres, slightly higher than the level of the catastrophic 1974 floods, by around 4:00am tomorrow.

The Brisbane River was a swirling torrent, with boats, pontoons and other debris being swept towards the sea and muddy water inundating low-lying areas.

By tomorrow morning almost 20,000 homes in the city will be flooded.

The floods have wreaked havoc on Brisbane's riverside infrastructure, with Lord Mayor Campbell Newman saying the city's CityCat ferry service has been "substantially destroyed".

The CityCat and CityFerry services could be out of action for months.

Raw sewage is spilling into the floodwaters after treatment plants were inundated by the rising waters.

Councillor Newman has urged people wading in the water to make sure they do not get any in their mouths.

Water in the city is safe to drink but residents have been asked to use it sparingly.

Council engineers are preparing to demolish Brisbane's floating RiverWalk along the New Farm cliffs, amid fears it could break up and send slabs of concrete floating down the river.

There were similar scenes to the west in the city of Ipswich, where homes and businesses in the CBD were submerged up to their roofs as the Bremer River approached a peak of 20.5 metres.

There were reports of looters and at least one bull shark being spotted in the flooded streets.

Ipswich Mayor Paul Pisasale said anyone found looting in the regional city would be "used as flood markers".

Around 115,000 people were without power across the region as Energex cut supplies in low-lying areas.

Earlier, the death toll from the floods rose to 12 after two more bodies were found in the devastated Lockyer Valley, which was hit by an "inland tsunami" late on Monday.

A man's body was found in a house at Grantham this morning and another at Lyon's Bridge nearby.

Fifty-one people were still missing as search crews scoured the area, but grave fears were held for only nine people.

There were no forced evacuations in Brisbane, where the CBD was a virtual ghost town this morning.

Bridges across the Brisbane River were closed ahead of the flood peak.

Public transport in Brisbane's CBD is operating at a minimum, with no in-bound bus services and trains running on a public holiday schedule.

Phone services are also struggling and residents are being urged to only use them if absolutely necessary.

Today residents in Brisbane suburbs were wading through shoulder-height water, frantically trying to clear out their homes and businesses or lending a helping hand to neighbours.

In the inner-city suburb of Rosalie, people formed a human chain trying to save stock from a local cafe before it became inundated.

As water crept up residential streets, furniture and other valuables were piled up high on verandahs and houses were abandoned.

Residents in Brisbane's Jindalee, Moggill and Yeronga areas have also been badly hit.

The Moggill ferry has broken its guidelines and although it is currently stable, assessors are on their way to potentially sink or demolish it to prevent it from posing any danger.

The Inner-City Bypass is now closed but all tolls on the Gateway and Logan motorways have been lifted.

Premier Anna Bligh says there are now 3,585 people registered in 57 evacuation centres in Brisbane and Ipswich.

Around 400 of those are in Brisbane but Ms Bligh expects that to swell to almost 3,000 overnight.

Releases from Wivenhoe Dam have been reduced to allow the Brisbane and Bremer rivers to drop.

The giant dam is at 190 per cent and is releasing 205 megalitres of water a day.

SEQ Water is expecting to increase flows once Brisbane's flood peak has passed.

Residents in the south-east are being asked to conserve household water use.

Authorities say the water is safe to drink but they would like to keep reservoirs full in case there are problems with treatment plants.

Grim task

In the Lockyer Valley, military helicopters joined search and rescue teams heading to Grantham and Murphy's Creek to search destroyed properties for bodies.

Ms Bligh says it is going to be a very hard day for the community.

"Families who are still holding out hope, some of them are likely to have their hopes tragically crushed," she said.

"I think it's going to be a tough and emotional day in the Lockyer Valley as those search and rescue teams get in there for the first time."

A telephone hotline - 1300 993 191 - has been set up for people seeking information on friends and relatives caught up in the flooding disaster.