Queensland water authorities have been found to have breached their duty of care over the 2011 floods in South-East Queensland, handing a victory to thousands of victims in a landmark class action.

Key points: Justice Robert Beech-Jones said decisions made by dam engineers ignored the operations manual they helped draft

Justice Robert Beech-Jones said decisions made by dam engineers ignored the operations manual they helped draft The judge found the engineers delayed to act, causing worse flooding

The judge found the engineers delayed to act, causing worse flooding Litigants' lawyer said the damages bill was likely to be in the "hundreds of millions"

The Supreme Court in New South Wales found flood engineers operating the Wivenhoe and Somerset Dams were negligent and failed to follow the manual they had helped draft.

While the court ruled in favour of the negligence claim against the Queensland government of the day, as well as Seqwater and Sunwater, other aspects of the case failed.

The class action alleged the dam operators failed to follow their own manual and did not make enough room for heavy rainfall until it was too late, heightening flood levels and damaging more properties.

Lawyers for the victims claimed the dam operators were careless and negligent and contributed to the deluge when dam engineers were forced to release large amounts of water to prevent the Wivenhoe Dam from collapsing.

Thousands of homes and businesses were inundated by flood waters in January 2011. ( Dave Hunt: AAP )

Maurice Blackburn Principal Lawyer Rebecca Gilsenan said it was an "enormous victory" for the 2011 flood victims.

She said "the damages bill will be very substantial" and likely in the "hundreds of millions".

"It is a landmark judgment in favour of the victims," she said.

"Our clients are pleased and extremely relieved at this outcome.

"This brings to an end a very long and difficult chapter to the lives of people in South-East Queensland."

Campbell Fuller from the Insurance Council of Australia said insurers paid the 2019 equivalent of more than $1.5 billion in claims following the 2011 Brisbane floods.

"They will review today's decision for its commercial implications," he said.

Queensland's Attorney-General Yvette D'Ath acknowledged the ruling and would "closely examine the judgement before making any comment on a possible appeal".

A man surveys the debris floating in the floodwaters inundating Ryan Street in the inner-Brisbane suburb of West End on January 12, 2011. ( ABC News: Giulio Saggin )

The judge ruled against the victims' claim of nuisance and trespass but the lawyers acting on their behalf said it had no bearing on the significance of the overall victory.

"The nuisance and trespass claim was a small claim at the back of the proceeding. It doesn't affect people's damages," Ms Gilsenan said.

Any payments that grants victims received after the floods would be subtracted from any damages they are awarded.

"I am satisfied that the legislature did not intend that a flood victim recover twice in respect to the same invoice of the cost item," Justice Beech-Jones said.

Lead plaintiff Vince Rodriguez, whose Fairfield business was destroyed after being inundated by floodwaters, said the decision had been a long time coming.

"The decision will allow me and my family to move on after what has been a long, trying time," he said.

One of the houses heavily impacted by flooding in 2011 at Auchenflower. ( Aimee: User submitted )

He said once the judgment sank in, it was "a great relief".

"It was an amazing feeling because it has taken almost nine years to come to this," he said.

Frank Beaumont, whose house at Goodna near Ipswich was destroyed in the flood, said the outcome was a win for the community.

"We should not ever have been flooded," he said.

"All I had left was a roof, three walls and the internal timber framework."

"I'm so pleased for everybody."

Rain of 'biblical proportions'

In handing down his ruling, New South Wales Supreme Court Justice Robert Beech-Jones said the rain that hit around January 11, 2011 was of "biblical proportions".

Justice Beech-Jones said the main argument in the class action was that calculations regarding the release of water from the Wivenhoe Dam were based "on the level of water in the dam and not the amount of rain forecast".

He noted the forecasts in the nine days leading up to the major flooding event "steadily increased and became ominous".

Goodna locals celebrate after watching the live stream of the decision on a laptop. ( ABC News: Talissa Siganto )

Justice Beech-Jones told the court many of the decisions made by dam engineers bore little reference to the instructions outlined in the operations manual.

He said all dam engineers committed a "breach of duty" during the operation of the dam from January 6 to January 11.

The dam engineers' mistakes derived from "a failure of approach, specifically a failure to follow the very manual they drafted 18 months earlier", according to Justice Beech-Jones.

The court heard the class action involved more than 2,100 pages of testimony and 26,000 documents, including 700 spreadsheets across 50 volumes.

During the floods, more than 2,000 homes in Brisbane and Ipswich were inundated and thousands more homes and businesses were affected.

Much of Queensland was declared a disaster zone, with 35 lives lost and approximately 200,000 people affected statewide.

The case was run in New South Wales because Queensland courts did not hear class actions when it was filed.