Brisbane's main defence from a catastrophic flood is facing an unprecedented test as heavy rain continues to inundate southern Queensland.

The Brisbane River has already broken its banks in part of the city and evacuations are underway in some suburbs.

Further upstream the Wivenhoe Dam, built in the wake of the deadly 1974 floods, is pumping record amounts of water over its spillway as the flood situation rapidly changes.

On Monday night releases from Wivenhoe's five gates were at about 236,000 megalitres, but that level was increased on Tuesday.

SEQ Water, which manages the dam, is warning that communities downstream are likely to be inundated until at least Sunday.

Wivenhoe Dam is already at 173 per cent and levels are rising. Its maximum capacity is 225 per cent.

Professor Hubert Chanson, a professor in hydraulic engineering at the University of Queensland, says the dam was built to withstand an event similar to what we are seeing.

But he says the unprecedented rainfalls southern Queensland has seen, combined with yesterday's 'inland tsunami' which swept through the Lockyer Valley, have pushed catchments to the limit.

"The last two or three days have seen a very unusual event in Toowoomba but if we look at the last two to three weeks there has been a lot of water flowing into the Brisbane River and until now we have been spared of any flooding in Brisbane thanks to the Wivenhoe reservoir," he said.

"I believe we are in an unusual situation where in Brisbane and Ipswich we might have some flooding that is linked with run-off in other parts of the catchment.

"In other words we have had a very unusual and deadly event."

'Catastrophic'

Professor Chanson says the dam operators have been progressively releasing water from the dam to prevent it filling.

"If the Wivenhoe become full up to the stage where the water in the reservoir is very close to the crest of the dam, it would mean a situation where the operators of the dam would have to open fully the gate of the spillway," he said.

"Anything coming into the reservoir would have to be immediately discharged."

He says it would be catastrophic if water made it over the top of the dam wall.

"The Wivenhoe Dam is not designed to be overtopped, it is what we call an embankment dam. And if water was to flow over the top of the crest of the dam, there would be a very high risk of erosion of the dam wall and ultimately failure of the dam," he said.

"If the water was to spill over the top of the crest of the dam we would look at the failure of the dam on the possible complete emptying of the reservoir into the Brisbane River with deadly consequences for the people living downstream."

But he says there are a number of safeguards to prevent such an event.

"The first one is the primary spillway, the main spillway you have seen on the TV news," he said.

"That is the location where they are currently controlling the downstream release of water into the Brisbane River.

"But they also have a secondary emergency spillway where if the water were to reach dangerous levels it would start to provide further release from the dam wall."

The Bureau of Meteorology says major flood levels in Brisbane are likely to be reached on Wednesday afternoon.