Australia's third-largest city Brisbane was Thursday spared the worst of floods that had threatened to overrun it, but destructive torrents sent a huge concrete walkway careering through the city.

Thousands of frightened homeowners were given an 11th hour reprieve when the churning Brisbane River that had been threatening the city with catastrophe peaked more than a metre (three feet) below the high that officials had feared.

The river, which runs through the centre of the besieged city of two million people, peaked at 4.46 metres (14.6 feet) at around 5:15am (1915 GMT Wednesday), more than a metre below the high officials had expected.

"Hydrologists ... believe it is at or near its peak," forecaster Brett Harrison after the churning river failed to breach the high of 5.4 metres it reached in floods that devastated the city in 1974.

But he warned: "We still expect it to be above major flood levels until sometime during Friday and remain high over the weekend."

The swollen river was already beginning to slowly recede, but the stricken and nervous city was reeling from damage wrought by the waters that have already deluged many areas of Brisbane.

A massive 300-metre stretch of a popular concrete walkway that was perched above the river and was popular with walkers and cyclists, was ripped from its moorings and was hurtling down the river past the city centre.

"Its staggering, the Riverwalk was a huge structure that we all know so well, but now it's just shooting down the river, smashing into everything in its path," an eyewitness told local radio.

Officials had planned to destroy the Riverwalk on Wednesday to avoid it becoming a waterborne missile, but conditions were too dangerous to carry out the operation.

Queensland state premier Anna Bligh had warned that residents would wake to "shocking" flood scenes.

"Brisbane will go to sleep tonight and wake up to scenes that they have, many of them, never seen anything like in their lives," she said late Wednesday.

But the river's failure to reach the expected peak of 5.5 metres is expected to have saved thousands of properties from flooding.

Officials had predicted that 20,000 properties would be completely submerged when the flood reached its projected peak of around 5.5 metres, and that more than 10,000 would be partially submerged.

But revised modelling based on a high flood peak of 4.6 metres showed that 11,900 properties would be fully flooded and 14,700 partially inundated -- fewer than feared, Mayor Campbell Newman said.

But thousands of other homeowners were not so lucky and some suburbs were under significant amounts of water, causing billions of dollars of damage.

"We all now have to rally together to help these people clean up, the ones that have suffered impacts," Newman said.

Damage was intense, with witnesses spotting entire houses in the river. The military considered scuttling a landmark ferry and a well-known restaurant that were in danger of floating away.

But residents of the city breathed a sigh of relief as they woke up to the news that they had been spared from the worst flooding since 1893.

"It was worse in '74, a lot worse," said John Mcleod, security director of the Stamford Plaza hotel, which lies near the Brisbane River in the city centre and which was forced to close due to flooding.

"I slept only one hour last night. We have 3.5 metres of water in the basement. The hotel will stay closed at least for another 10 days," he told AFP.

Police shouted to onlookers eager to assess the flood levels for themselves to keep clear of inundated streets as sirens and alarm bells wailed in the distance.

The death toll from flash floods upstream from Brisbane around the town of Toowoomba earlier this week rose to 13 overnight, with scores of other people unaccounted for.

Rescuers were picking through the wreckage of hamlets searching for the dead and missing.