Across 30 broadcasts between October 2014 and August 2015, Mr Jones made defamatory claims about John, Neill, Denis and Joe Wagner, implying they were responsible for the deaths of 12 people in the 2011 Grantham floods in south-east Queensland.

Justice Peter Flanagan handed down a 350-page verdict on Wednesday to a packed courthouse, awarding $938,746 to each of the Wagner brothers and describing Mr Jones’ words as “vicious” and “spiteful”.

Mr Jones did not attend the verdict, but issued a statement saying he needed to "read the entire judgment very carefully".

"I do feel that I have somehow let down the people of the Lockyer Valley who sought my assistance," he said.

"As the judge found in this case, I sought to ventilate concerns expressed to me by those people.

"I am restrained in what I can say because I am now subject to injunctions and will be making no further comment.”

Mr Jones and the two radio stations have been permanently banned from mentioning the family or any of the matters brought before the court.

The Sydney radio host and Macquarie Media, the owner of the two radio stations, are considering their options to appeal their loss.

4BC and 2GB are owned by Macquarie Media, which is majority owned by Fairfax Media.

On his breakfast shows, Mr Jones claimed that the Wagner family was part of a "massive cover-up" to avoid responsibility for the deaths during the floods.

Mr Jones claimed the deaths were attributable to the collapse of a dam wall at a quarry owned by the Wagners in the Lockyer Valley in south-east Queensland. He also claimed the four brothers were corrupt, bullies and liars.

The influential presenter made defamatory claims that the family had covered up their "culpability" in the flood deaths and had lobbied Queensland Premier Anastasia Palaszczuk to avoid exposing the "truth" in the following Senate inquiry.

He also accused the brothers of being corrupt in their dealings as they built an airport at Toowoomba.

In the detailed judgment, Justice Flanagan said Denis Wagner had described the "gut-wrenching" feeling of hearing Mr Jones say the Wagner brothers were responsible for the flood deaths.

All four brothers detailed the deep anger, humiliation, embarrassment and hurt they felt over the months of Mr Jones' commentary.

The brothers were seeking $4.8 million in damages in a seven-week judge-only trial that began earlier this year.

On Wednesday Justice Flanagan said that prior to the broadcasts made by Mr Jones, the Wagner brothers had an excellent reputation for honesty and integrity throughout the community.

“The defamatory broadcasts have caused each of the plaintiffs to suffer profound personal hurt and harm to their reputations, which includes their business reputations,” he said.

The long-established Toowoomba family, all of whom were at court on Wednesday, expressed their relief that Justice Flanagan had favoured their complaint, saying they had faced vilification from Mr Jones since 2011.

"Until we commenced this action, Mr Jones' malicious attacks on our character and that of our family were unrelenting,” Denis Wagner told the media outside the Brisbane Supreme Court.

"Mr Jones and his defendants relentlessly misled their listeners and the people who entrusted them to be honest and truthful.

"They did this at great expense to our family and to the Toowoomba community. We decided to take a stand against this abhorrent, vicious and deceitful, spiteful behaviour."

Defamation proceedings also brought against journalist Nick Cater were dismissed by Justice Flanagan, who found Mr Cater had not sought to convey defamatory imputations against the Wagners.

Radio 2GB and Mr Jones must pay each of the Wagner brothers $750,000 plus interest of $78,102.74, while 4BC and Mr Jones must pay each of them $100,000 with interest of $10,643.84.

The total $3.75 million payout is one of the largest on record, and follows a similarly high $4.6 million payout made to actress Rebel Wilson in a defamation case that was later reduced on appeal.