Broadcaster Alan Jones, 2GB and 4BC defamed the Wagner family in a series of radio broadcasts between 2014 and 2015, the Supreme Court in Brisbane has found.

Key points: Jones, 2GB and 4BC defamed the Wagner family when they alleged the brothers were responsible for the deaths of 12 people

Jones, 2GB and 4BC defamed the Wagner family when they alleged the brothers were responsible for the deaths of 12 people Jones was motivated by a desire to injure the brothers' reputations, the judge found

Jones was motivated by a desire to injure the brothers' reputations, the judge found Denis Wagner said Jones' "attacks" on the family's character were unrelenting

The Toowoomba-based Wagner family has been awarded around $3.75 million in damages.

Following a seven-week judge-only trial, Justice Peter Flanagan found 2GB and Jones published 27 broadcasts which conveyed "extremely serious" defamatory allegations.

Those allegations included that the Wagner family were responsible for the deaths of 12 people, including two children, in the 2011 Grantham floods when a quarry wall owned by the family collapsed.

The court also found the broadcasts alleged that the brothers had illegally built the Wellcamp Airport and had stolen airspace from the Oakey Army Base.

Jones and 2GB tried to argue many of the allegations made by the broadcaster were substantially true and others were a fair report of the Grantham Flood Inquiry.

The Wagners claimed Jones implied they were responsible for the deaths of 12 people in the 2011 Grantham floods. ( AAP: Glenn Hunt )

Justice Flanagan found that in separate broadcasts on 4BC in February 2015, Jones made allegations that each of the four brothers tried to persuade Premier Annastacia Palaszczuk that any suggestion they'd covered up the deaths of 12 people was a "conspiracy theory".

Jones also alleged the brothers were corrupt businessmen.

Jones and 2GB will be required to pay the Wagner family more than $3.3 million. In addition, Brisbane station 4BC and Jones were also ordered to pay the family more than $440,000.

Journalist Nick Cater was also sued but the defamation claims against him were dismissed.

Jones' comments were 'vicious and spiteful': Judge

In his 253-page judgement, Justice Flanagan said Jones was motivated by a desire to injure the brothers' reputations.

He said Jones' conduct was unjustified because of "the intrinsically vicious and spiteful wording used … [and] Mr Jones' wilful blindness to the truth or falsity of the defamatory accusations".

Justice Flanagan said Jones' argument that he did not hate the Wagners was wholly contradicted by the content in the broadcasts and in email exchanges to Qantas chief Alan Joyce.

In 2014, Jones wrote to Mr Joyce to express his disgust the airline had agreed to fly into the Wagner-built Wellcamp Airport in Toowoomba.

Mr Joyce told Jones their offer was commercially attractive, to which Mr Jones wrote back: "Alan, I fear we're on a collision course. Of course these people will bribe you to go there."

Justice Flanagan said the seriousness of the defamation meant there needed to be "a substantial award of damages to signal to the public the vindication of the plaintiff's reputations".

Members of the Wagner family arrive in court in May 2018. ( ABC News: Allyson Horn - file photo )

Speaking outside court Denis Wagner said he and his family had faced "vilification" by Jones for seven years.

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

"The comments by Alan Jones have been described by the judge as unjustifiable.

"We decided to take a stand against this abhorrent, vicious and deceitful, spiteful behaviour.

"Justice Flanagan has delivered a judgement which has clearly indicated that people regardless of how much influence they may consider they have will be held accountable for their words and their actions."

In his judgement, Justice Flanagan described each of the four Wagner brothers breaking down during the trial.

"Having observed each plaintiff give evidence, I find that the defamatory broadcasts have caused each of them to suffer profound personal hurt," Justice Flanagan said.

"Denis Wagner experienced anxiety … he found the accusations of bullying and intimidation to be 'gut-wrenching' and felt 'humiliated'," he said.

Macquarie Media CEO Adam Lang said 2GB was disappointed by the decision.

"2GB, 4BC and Mr Jones will be considering carefully their appeal options," Mr Lang said.

In a statement Jones said he needed to read the 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."

Jones denied having 'bitter hatred'

Throughout the trial, the Wagner family's lawyer Tom Blackburn accused Jones of making "grossly irresponsible" allegations.

In cross-examination, Jones denied he had a "bitter hatred" of the family and was merely dealing with the facts as he saw them.

A railway crossing at Grantham is littered with debris from floodwaters on January 12, 2011. ( AAP: Dave Hunt )

A commission of inquiry was launched following the deaths of the 12 people after flash flooding in Grantham and exonerated the Wagners from any responsibility, after it examined a quarry that had been developed by the prominent Toowoomba family.

In October 2015 after the findings were released, Commissioner Walter Sofronoff said quarry owner Denis Wagner and his family had been unfairly targeted as the culprit for the flood.

"I think they were unjustly blamed by some people and I think they were viciously blamed by some elements of the media and they shouldn't have been," he said.

Under cross-examination by Mr Blackburn, Jones maintained his belief that the quarry was responsible for the deaths.

"I've said that many times, that's not news," Jones told the court.

"You had no hydrological evidence when you made this statement of fact on this day?" Mr Blackburn asked.

"No, I did not," Jones replied.