It's been almost nine years since Queensland retiree Frank Beaumont watched floodwaters swallow his home west of Brisbane.

He always believed he was a victim of negligence. On Friday a court finally agreed in a landmark ruling that saw tears flow for all that he has endured.

The 77-year-old is one of 6800 flood victims who should get compensation in the new year after a successful class action against the state government and the operators of Wivenhoe and Somerset dams.

The NSW Supreme Court has ruled they were failed by engineers who negligently operated infrastructure built to save them from the kind of “biblical” deluge that fell upstream of Brisbane and Ipswich in January 2011.

Justice Robert Beech-Jones found claimants suffered worse flood damage because engineers did not properly manage the two dams during an extraordinary rain event.

“The mental stress has been horrendous,” Mr Beaumont has told AAP.

Shirtless and wearing the same old pants he had on when his Goodna home went under, the 77-year-old admits to being surprised by his own tears on learning he’d won.

“It sort of got to me. We’ve had so many trodden down moments where the insurance didn’t pay, being kicked out of a rental home and then having to rebuild an absolutely devastated house.”

Justice Beech-Jones found dam operators did not follow their own operation manuals by failing to act on rainfall forecasts.

Some of the 23,000 homes and businesses hit by floodwaters would not have been hit as hard if not for the acts of negligence, the court found.

Maurice Blackburn principal lawyer Rebecca Gilsenan says the case is one of the biggest class action decisions in Australian legal history.

She said lawyers would not translate the findings into financial damages before the case returns to the court in February.

But she said damages could could total in the “several hundreds of millions” of dollars.

The focus from the state government should be on securing timely compensation, and on making sure Queenslanders were never again at the mercy of mismanaged dams, Ms Gilsenan said.

Justice Beech-Jones was scathing of engineers’ negligent management of the dams and their failure to factor in rainfall forecasts in deciding how best to respond to the flood event.

He said although a flood event was declared on the morning of January 6 and solid rain continued throughout that day with much more rain forecast, water releases did not start until the afternoon of January 7.

He said “the heavens opened” on January 9, when rainfalls of up to 400mm fell in catchment areas above the dams. By January 11 thousands of buildings went underwater.

Justice Beech-Jones said “the distress, dislocation and heartache that was occasioned by the flooding was taken as a given in the proceedings”.

The case has been heard in NSW because at the time it was lodged, class actions were not possible in Queensland.

The successful class action was taken against Seqwater, SunWater and the state government, who employed the four engineers who managed the two dams during the flood event.

Queensland Attorney-General Yvette D’Ath said the government recognised the decision, but it was too early to say whether planned to appeal.

“The government will closely examine the judgment before making any comment on a possible appeal,” she said in a statement.