Wieambilla police killers' father reveals estrangement following child ...

Australia's first alleged Christian terrorists forged their fateful bond decades ago when they jointly accused two of their parents of child sexual abuse and cut all family ties.

Nathaniel, Gareth and Stacey Train died in a shootout with Queensland police after killing three people, including two officers, on a remote property at Wieambilla on the Darling Downs in December 2022.

Police have concluded the trio "executed a religiously motivated terrorist attack", believing the world was ending and police were "monsters and demons".

How the trio reached this point will be examined in an upcoming coronial inquest into the shooting.

Gwen and Ron Train, with two of their four children, Gareth and Nathaniel.(Supplied: Train family)

A clue has been offered by Nathaniel and Gareth's father in an interview with ABC's Background Briefing.

Ron Train has revealed for the first time to Background Briefing that his sons cut contact in 1999 after alleging they were abused as children.

Estranged from the family, the trio then moved around regional Australia, frequently sharing a home.

In the last year of their lives, they were isolated and paranoid, culminating in the police shooting.

Ron Train says Gareth and Nathaniel's abuse allegations left the family bewildered.(Background Briefing: Nathan Morris)

Ron Train said he and his late wife, Gwen, were horrified by the allegations against them.

"That was the devastating thing to me, to think these boys who I had raised and loved … made certain allegations which were totally false and unproven," he said.

Ron and Gwen's other children stuck by their parents following the estrangement.

Nathaniel Train and Stacey Christoffel met as teenagers through a Baptist church in Toowoomba.(Supplied: Train family)

The Train brothers met Stacey Christoffel as teenagers through a Baptist church in Toowoomba, where their father, Ron, was a minister.

Ron Train said Stacey took the lead in ventilating the abuse claims during a family wedding in Coffs Harbour in 1999.

She and Nathaniel, who were then married with two young children, turned up unexpectedly with Nathaniel's brother, Gareth, after avoiding the family for months.

"Stacey blew her top and made certain accusations against Gwen, which bewildered Gwen," Ron Train said.

"She flew off the handle. She was very aggressive.

"I heard all the commotion, and my brother got up and went out to find out what it was. And he asked Stacey to leave the house because of her behaviour."

Nathaniel and Gareth told Ron's brother they'd been "sexually abused" at home in Richmond, north-west of Sydney, where the family lived until the boys were aged six and seven respectively.

"My brother said to them, 'Don't be ridiculous. You know your mother and father loved you very much … and wouldn't even contemplate such things,'" Ron recalls.

Ron said Nathaniel tried to begin a "rational discussion" with him but "Stacey flew in and … pulled him away".

Stacey Train is carried by Gareth (left) and Nathaniel (right) on her wedding day.(Supplied: Train family)

He said he later wrote to his sons, challenging their claims and suggesting they be put to the family doctor, but the brothers never responded.

Ron said he asked about the abuse allegations when meeting a Queensland Police detective investigating the 2022 shootings.

"[The detective] said: 'We've investigated all that stuff. There's nothing in that at all.'"

Queensland Police told Background Briefing it was not appropriate to comment on matters relevant to a coronial inquest, with hearings beginning July 29.

A relative told Background Briefing the broader family did not consider the abuse allegations credible.

This relative said Nathaniel accused his mother of harming her children by walking around the house in her underwear.

Ron Train said he believed Stacey had a "major influence" on the Train family upheaval.

Stacey Train on her wedding day.(Supplied: Train family)

She had been "deeply troubled" by her own family's handling of a suspected sexual predator within its ranks, he said.

She was distressed to learn her parents had invited a relative accused of sexual abuse to her 1995 wedding without informing her of the allegations.

Ron says Stacey wanted her parents to be publicly castigated in the Church where he was a minister.

"And I said, 'No, I'm not going to put your parents in an embarrassing situation'.

"Well, that didn't go down well with Nathaniel and Stacey."

He said the couple also pressured him to reveal what he had known about the alleged abuse.

"I knew some of the circumstances, but because it had been shared to me in confidence as a pastor, I couldn't share any information, and that disturbed [Stacey and Nathaniel]," he said.

Stacey's father, Philip Christoffel, declined to comment.

Ron Train, a Baptist pastor, was left devastated by the family estrangement.(Background Briefing: Nathan Morris)

Mr Train said the trio's decision to "sever ties with both our family and Stacey's family was devastating".

"I thought, that's a big thing to do, to walk away from two families and try to develop themselves, the three of them — Stacey, Gareth, Nathaniel."

Ron would not see his two grandchildren again for decades.

Following the estrangement, Nathaniel left his job at a high school with an elite sports program near the Gold Coast, and moved with Stacey and Gareth to north Queensland.

Nathaniel and Stacey began carving out careers as teachers in Indigenous education.

It eventually filtered back to the families that Stacey had left Nathaniel for Gareth.

This was deeply at odds with their conservative Christian upbringing.

Stacey and Gareth then primarily raised Nathaniel's children, who later went to boarding school.

"[Gareth] wanted to be head of his own family. That was his dream," Ron said.

"He achieved it, but at the expense of Nathaniel and Stacey's marriage, in my view.

"I see it as an absolute betrayal."

Gwen and Ron Train tried to reconcile with their two estranged sons.(Background Briefing: Nathan Morris)

Despite Gareth replacing Nathaniel as Stacey's husband, the trio lived together on and off and bought a house on the Atherton Tablelands in 2004.

Ron said when Stacey's parents tried to reconcile with her that year: "Gareth came to the door with a baseball bat and said, 'Get out of here.'"

Ron said he took his own shot at reconciliation the following year, with his wife, Gwen.

"Gareth and Stacy were inside somewhere," he recalls.

"Nathaniel answered the door. I said, 'Can you speak to Mum and I, mate,' and he said, 'No, Dad. Off the property, please.'"

Ron said that even before the estrangement, Nathaniel and Gareth were the closest of his four children despite their starkly different personalities.

Ron and Gwen Train with their four children, including Gareth and Nathaniel in front.(Supplied)

Nathaniel, a junior tennis champion who achieved at school, was a "calming influence" and "on track to help people".

Gareth was "more aggressive, controlling" and had learning difficulties. He was expelled from an agricultural college in Dalby after throwing another student through a pane of glass.

Nathaniel and Gareth applied to join the Australian Defence Force, but abandoned the plan when Gareth "failed the psych test", according to Ron.

The estrangement brought the brothers even closer, forging a bond that survived Gareth taking Nathaniel's wife.

Stacey and Gareth Train in a video posted to YouTube on the night of the police shooting at Wieambilla, in rural Queensland.(YouTube)

Ron believes Nathaniel initially stayed close to his brother and ex-wife "to have contact with his children".

But he suspects the connection ran deeper than that.

He could not picture Nathaniel buying into unhinged conspiracy theories like Gareth and Stacey, who were "two people extreme in their ideas".

But ultimately, Nathaniel was "certainly under the influence of Gareth and Stacey to support them through whatever reckoning was going to take place".

"There was certainly a loyalty bond between Gareth and Nathaniel, which is — I don't know whether it's [implicitly] disturbing, but in the end, it worked out that way," Ron said.