In total, Mr Jones and radio stations 2GB and 4BC will have to pay $938,746 in damages to each of the brothers. The broadcaster and radio stations would consider options to appeal, Macquarie Media said in a statement.

The influential radio broadcaster, journalist Nick Cater, Harbour Radio and 4BC were all being sued for $1.2 million respectively, over comments made by Mr Jones on his radio program about the collapse of a dam wall at a quarry owned by the Wagner brothers during the 2011 floods.

Defamation proceedings against Mr Cater were dismissed.

John, Denis, Neill and Joe Wagner took the radio host and radio stations to court, claiming they were being named as responsible for the deaths of 12 people in the 2011 Grantham floods when the wall of a quarry the brothers owned in the Lockyer Valley collapsed.

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

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

Justice Peter Flanagan handed down his verdict at Brisbane Magistrates Court on Wednesday morning to a packed courthouse with family of the Wagner brothers attending. Mr Jones did not attend the verdict.

Justice Flanagan said the 76 defamatory imputations brought before the court were "extremely serious" and of the "gravest kind".

The imputations made by Mr Jones on his radio show included claims that the Wagner family was part of a "massive cover-up" to avoid responsibility for the deaths.

Over 30 broadcasts between October 2014 and August 2015, Mr Jones 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".

Further, Mr Jones accused the brothers of being corrupt in their work to build an airport at Toowoomba.

Justice Flanagan found that 4BC and Mr Jones had sought to defend their claim over the airport construction, but had failed to prove it was substantially true.

Speaking to media on behalf of the Wagner family outside the Brisbane Supreme Court after the verdict, Denis Wagner said the verdict had given the family "enormous confidence in the justice system".

"Our family, and us as individuals, have faced vilification from Mr Jones since 2011," Mr Wagner said in a prepared statement.

"Until we commenced this action, Mr Jones' malicious attacks on our character and that of our family were unrelenting.

"The Wagner family has been part of the Darling Downs and Toowoomba community since the mid-1850s.

"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."

Mr Wagner said his family had not gone into the defamation proceedings in search of financial gain, and had not yet had time to consider the multimillion-dollar payout.

Mr Wagner said he and his brothers had only asked for an apology from Mr Cater but had not received it, hence included him in the defamation proceedings. He said the family would consider their options following the verdict.

In a statement, Macquarie Media said 2GB was disappointed by the decision and the radio stations and Mr Jones would be considering options for appeal.

"2GB, 4BC and Mr Jones will be considering carefully their appeal options. As injunctions now restrain them, nothing further can be said at this time," the statement said.

Mr Jones also issued a statement saying he needed to "read the entire judgement 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.”

Late in the trial, Mr Jones' lawyer Robert Anderson, QC, had dropped an "honest opinion" defence, conceding that some of the comments made by Mr Jones could not be defended.

"We accept there must be awarded damages because as your honour has seen ... there are some meanings that are not able to be defended," Mr Anderson told the Brisbane Supreme Court during closing submissions in June.

On Wednesday, before releasing his 350-page final verdict, Justice Flanagan found that Mr Jones, for whose conduct 2GB and 4BC were "vicariously liable", had engaged in unjustifiable conduct and was "motivated by a desire to injure".

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

The payout is believed to be one of the largest in Queensland history and follows a similarly high payout made to actress Rebel Wilson, which was reduced on appeal.

- with AAP