Christopher Dawson murdered his wife in January 1982 because he was so tortured at the thought of losing his teenage lover, a judge has found.
In an almost five-hour judgment read to two packed courtrooms on Tuesday, Justice Ian Harrison found Dawson guilty of murder - concluding a 40-year-old mystery and leading to jubilant applause and cheering from those listening.
"I am left in no doubt. I am satisfied beyond reasonable doubt that the only rational inference (is that) Lynette Dawson died on or about 8 January 1982 as a result of conscious or voluntary act committed by Christopher Dawson," the NSW Supreme Court judge said.
The former PE teacher was "so distressed, frustrated and ultimately overwhelmed" that he could not have an unfettered relationship with his teenage babysitter, known as JC, that he plotted to kill his wife and install his young lover in his family home days after the murder.
Justice Harrison rejected the possibility Mrs Dawson abandoned her husband and children to vanish without a trace from their home on Sydney's northern beaches.
"The proposition is ludicrous," he said.
Mrs Dawson had a strong attachment to her daughters and had remained steadfastly committed to her husband even as her marriage crumbled around her.
Addressing the media outside court, Greg Simms said his sister's name had been cleared, and that she had been betrayed by a man she loved. But she was still missing.
"We would ask Chris Dawson to find it in himself to finally do the decent thing and allow us to bring Lyn home to peaceful rest," he said.
Hedley Thomas, the journalist behind The Australian's Teacher's Pet podcast on Mrs Dawson's disappearance, said he felt privileged to be able to help in the way that he could.
"Her story struck me as so unjust, so unfair at the time," he said.
He criticised the original police investigation for not taking proper action initially, saying that Dawson should have been charged 40 years ago.
"He has had 40 years of his life that he has been able to enjoy without any accountability. That's disgraceful."
Paul and Peter Dawson refused to comment on their brother's conviction, swearing at TV cameras as they left the court.
Despite the case being wholly circumstantial, without a body or murder weapon, Justice Harrison found there was no other reasonable hypothesis for the disappearance other than murder given Dawson's growing obsession with JC prior to the killing and how quickly she had been installed in his home.
JC and the former Newtown Jets rugby league player eventually married in 1984 and separated in 1990.
The babysitter's evidence was mostly truthful and reliable, Justice Harrison said, rejecting Dawson's allegations she had been corrupted by an acrimonious custody battle between them.
However, claims by JC that Dawson had driven her somewhere in 1981 to find a "hitman" to kill his wife were dismissed as improbable.
Justice Harrison dismissed claims Mrs Dawson had been seen alive after January 1982 as fabrications, unreliable or too vague to be of any worth. He also rejected claims by Dawson that he had been contacted by his wife after her disappearance as outright lies.
Dawson was previously a man of good character and there was no evidence he had been physically or verbally violent towards his wife before the murder.
Some evidence of allegedly violent incidents witnessed by friends and neighbours were rejected as being infected by the publicity surround the case.
This included through The Teacher's Pet podcast with Thomas speaking to individuals who would become crucial witnesses in the case and contaminating their memories by revealing what others had said.
The judge called the original police investigation into what happened "lackadaisical" and "arguably less than perfect" but said Dawson had not pointed to any significant disadvantage he had suffered because of the delay.
Dawson will apply for bail on Thursday, pending a sentence hearing. It is not known whether he will appeal the verdict.
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