Alan Jones took on a wealthy family from his native Toowoomba, alleging they were responsible for the deaths of 12 people.
The influential Sydney broadcaster, his radio station 2GB and Brisbane's 4BC have been hit with a record defamation payout of $3.7 million.
They claimed the Wagner family was responsible for the deaths in the 2011 Lockyer Valley floods when a wall of a quarry they owned collapsed, allowing a "tsunami"-like wave to consume the town of Grantham.
Jones and his co-defendants could not defend the defamatory comments, labelled "extremely serious and of the gravest kind" by Brisbane Supreme Court Justice Peter Flanagan on Wednesday.
The judge said comments had ruined the Wagners' good name.
"Prior to the publication of the defamatory broadcasts, each plaintiff enjoyed an excellent reputation for honesty and integrity," Justice Flanangan said.
"The defamatory broadcasts have caused each of the plaintiffs to suffer profound personal hurt and harm to their reputations."
Jones began his attack on the Wagners on 2GB in October 2014, when he claimed a high-level cover-up of their involvement in the deaths.
He took to the airwaves about 30 times to discuss the matter, describing Grantham as "one of the most corrupt metaphors in Australia".
"This was not an act of God. This devastation was brought about by human intervention. It could have been avoided," he said in one broadcast.
After six months, Jones relented.
But in that period listeners would have been swayed by Jones' "determined, indignant and accusing" tone, Justice Flanagan ruled.
"The ordinary reasonable listener would have been left with the impression that the deaths of people at Grantham had not been investigated and that the truth had been suppressed," he wrote in his judgment.
Toowoomba brothers John, Denis, Neill and Joe Wagner had sought $1.2 million each from the three defendants plus veteran journalist Nick Cater, who had the complaint against him dismissed.
The $3.7 million in damages smashes the previous record Australian defamation payout to Perth barrister Lloyd Rayney.
Mr Rayney was awarded $2.62 million last year after police defamed him by naming him as the "prime" and "only" suspect in the murder of his wife, Corryn.
Justice Flanagan noted an independent inquiry into the floods by Walter Sofronoff QC had found the deaths would have occurred from the sudden surge of water into Grantham irrespective of the quarry.
He also deemed defamatory the claim the Wagners had put their interests ahead of the nation's by "illegally" building their Wellcamp Airport in Toowoomba and were able to do so due to political access.
Denis Wagner says he does not expect an apology from Jones for being "vilified".
"Mr Jones and his co-defendants relentlessly misled their listeners and the people who had trusted them to be honest and truthful," he said outside court.
"We decided to take a stand against this abhorrent, vicious, deceitful, spiteful behaviour."
Jones and the stations are considering an appeal.
"I do feel that I have somehow let down the people of the Lockyer Valley who sought my assistance," he said in a statement.
"As the judge found in this case, I sought to ventilate concerns expressed to me by those people."
Australian Associated Press