Chris Dawson's lawyer has told a judge her client's first wife may have abandoned her family and subsequently met with misadventure, evoking the disappearance of former prime minister Harold Holt.
Mr Dawson, 73, has pleaded not guilty to murdering Lynette Dawson, who went missing from Sydney's northern beaches in 1982.
As his trial in the NSW Supreme Court draws to a close, barrister Pauline David has outlined why Mr Dawson's legal team argues there is an alternative hypothesis consistent with innocence.
Ms David cited evidence heard during the proceedings of alleged sightings of Ms Dawson after she was reported missing, the last of which was in 1984.
"At some point in time she has potentially created a new life, she has subsequently passed away, she has met with misadventure by some other way," she told the judge.
Ms David said there was also the "deeply unpleasant" possibility Ms Dawson took her own life and her body was not found.
She suggested a person's body may not be found if the death involved the sea.
"Harold Holt ... nobody knows where he went," she said.
"We can't say that happened, we don't know."
The Crown case is that Mr Dawson murdered his wife and disposed of her body as he pursued the family's babysitter, known as JC, who was a student at the high school where he taught.
Crown Prosecutor Craig Everson has previously said Mr Dawson was "besotted" with the student and, following four failed plans to leave his wife and be with JC, he resorted to murder.
Ms David accepted it was Mr Dawson who placed Ms Dawson into the "terrible mindset" of serious contemplation about her life.
She said if a person was to abandon their family, they may not want input from relatives about the decision.
Ms David admitted the fact that all of Ms Dawson's clothes remained at their home was a "curious aspect" of the case.
But she argued that if, as the Crown suggests, Mr Dawson had the capacity to kill Ms Dawson alone or with assistance and dispose of her body, it was "nonsensical" that he would leave the clothes intact rather than pack a suitcase to make it look like his wife left.
Ms David submitted Mr Dawson was an honest, open man.
She also took aim at the "narrow" police investigations into the disappearance.
"We say there has been a consistently poor attention to detail for the signs of life inquiries," she said.
The judge was told changing a name and opening a bank account 40 years ago were "entirely different exercises" than they are today.
The trial continues.
