Barrister Julian Sexton QC, for the plaintiffs, said operators of Wivenhoe dam, which was largely built to protect Brisbane from flooding, failed to free up space for huge downpours in early January 2011 because they were "unreasonably concerned" with maintaining water supply after years of drought.
Their early focus was also misplaced on keeping rural bridges open and avoiding "minor" urban flood damage, ignoring the risk of a major flood when the dam overflowed just as river levels were peaking in Brisbane and Ipswich, Mr Sexton said.
"Waiting for certainty is not the language of reasonable care, which focuses on probabilities, even possibilities, not on being certain that harm will occur before taking reasonable precautions," he said.
"[Dam engineers] in effect bet that the rain which had already fallen would be all the rain that would fall and bet against the four and eight-day forecast.
"The consequence was that they failed to create sufficient storage ahead of peaking flows that came on 10 and 11 January and thus had to make a very large release on 11 January which corresponded with peak flows downstream thus causing ... greater flooding."
Use left and right arrow keys to control image transition 2016: A street in the inner-Brisbane suburb of Rosalie. (ABC: Tim Leslie) 2011: Kayakers explore a flooded street in Rosalie. (AAP: Dave Hunt)
Mr Sexton said by January 11, when the dam's fuse plugs risked bursting and causing uncontrolled torrents of water flowing into the Brisbane River, the dam engineers were forced to take "drastic action", dumping about 7,400 cubic metres of water a second, worsening floodwaters already peaking downstream.
He said smaller releases on January 7 were "too late and too little".
Dam operators justified their actions by arguing that four to eight-day forecasts were too unreliable and water should only be released from dams "when rainfall has actually occurred so the effect of flood mitigation ... is known with certainty," he said.
Mr Sexton said this "mindset" ultimately left them with no time to clear space for adequate storage of flooding rains upstream of Queensland's capital - and defied both "common sense" and the flood manual under which the dams operated.
The plaintiffs argued that the belated increase in water releases in the half day up to 7:00pm on January 11 caused greater flooding than if engineers had started releases earlier.
Many property owners were left high and dry by insurers in what was the worst flood to hit the southeast since 1974. PHOTO: The class action has started almost seven years after the disaster in January 2011. (Nathalia Rayfield: User submitted)
Mr Sexton said he would call dam experts who would underline the importance of "pre-empting" rainfall to clear up storage space and halting releases altogether in flood peaks.
The misplaced "mindset" of the engineers was epitomised in an Seqwater flood summary report for January 9 to 10 that noted a "a full awareness of rainfall forecasts and associated potential flood impacts".
In a statement, Seqwater acting chief executive Donna Gregory said the operator was "confident" the court would find it had in fact "significantly reduced the impact of the flood".
"The magnitude of the rainfall meant it was inevitable that Brisbane would flood," she said.
The class action case will hear opening submissions for two weeks, before adjourning until next year when witnesses will be called. PHOTO: Many property owners said they had been left high and dry by insurers. (Giulio Saggin: ABC News)
PHOTO: Homes in Ipswich covered by floodwaters on January 12, 2011. (AAP: Dave Hunt)
PHOTO: More than 6,000 people have joined the class action that is being heard in the Supreme Court in Sydney. (User submitted: Chris McDowall)
2011: Kayakers explore a flooded street in Rosalie. (AAP: Dave Hunt)
PHOTO: The class action has started almost seven years after the disaster in January 2011. (Nathalia Rayfield: User submitted)
PHOTO: Many property owners said they had been left high and dry by insurers. (Giulio Saggin: ABC News)
