Dam operators 'too late' in Qld flood disaster, court told

DAM operators failed to act with "simple common sense" during the 2011 Queensland floods causing unnecessary disaster across the state, a court has heard.

The actions of the operators of the Wivenhoe and Somerset dams during the catastrophic floods are under the spotlight in a class action -- involving more than 6000 claimants -- currently underway in the NSW Supreme Court.

Maurice Blackburn Lawyers are representing the thousands of impacted people and businesses in the legal action, claiming the dam operators failed to follow their operations manual.

In his opening address on Monday, counsel for the plaintiffs, Julian Sexton SC, said flood engineers at the time were "blinkered" by several factors, which contributed to far greater flooding than would otherwise have occurred.

Mr Sexton said that on January 11, 2011, the peak of the rainfall,  the rate of release at the Wivenhoe dam, in the state's south-east, was more than 7,400 cubic metres per second, a dramatic increase from about 1,400 cubic metres per second the day before.

He said that was because engineers at the time unreasonably ignored the region's 4-8 day forecast, and were overly concerned about potential flooding in rural areas and minor flooding in Brisbane.

They were also more concerned than they should have been about maintaining dam supply levels, he told the court.

"They bet against the 4-8 day forecast being correct," Mr Sexton said.

"It was not only contrary to the manual it was contrary to common sense.

"It's the defendants in the case who are arguing against common sense."

When the engineers eventually did take action it was "too late and too little", causing far greater flooding than would have been the case if releases occurred earlier and in a "more timely fashion", Mr Sexton said.

"It was a matter of simple common sense" to take precaution against "catastrophic harm," he added.

Mr Sexton said that during the trial -- listed to run for two weeks this year -- the court would hear from a great many witnesses and that scientific evidence would also be adduced.

The trial began on Monday morning in a packed courtroom including a significant media presence and 12 barristers representing the various parties, including the Queensland Government.

The floods in early 2011, which are the subject of the long-awaited hearing, caused hundreds of millions of dollars in damage in Brisbane and southeast Queensland.

On her way into court, Maurice Blackburn class action principal Rebecca Gilsenan said a key issue would be whether the manual was followed in what she expected be an emotional case for her clients.

"They didn't create storage space as a precaution ... the result of that was that they had to open the gates and allow huge amounts of water out of the dam at once," she told reporters.

Seqwater, who is defending the civil action, said it was confident its management of Wivenhoe and Somerset dams would be vindicated.

"We have never wavered from our belief that our flood engineers did an extraordinary job in the most difficult and demanding circumstances. This position has not changed," it said in a statement.

"The January 2011 event was one of the most damaging and extreme natural flood disasters ever experienced in our region. Wivenhoe Dam received inflows much greater than the 1974 flood."

The trial continues before Justice Robert Beech-Jones.