The "salvage and exploration club" diary was kept by one boy, and contributed to by another, tracking their adventures diving off the Adelaide coast that summer.

The dives regularly involved one of their fathers, Allan "Max" McIntyre, and family friend Anthony Alan Munro, who will be sentenced in August for abusing boys in 1965.

One of Munro's victims and a contributor to the diary was Andrew McIntyre, who is now aged 63.

Mr McIntyre says the diary is evidence that his father and Munro were frequenting Glenelg beach in the days before Jane, Arnna and Grant Beaumont went missing there on Australia Day 1966 in one of the country's most infamous mysteries.

The children were aged nine, seven and four when they took the bus from their nearby home to spend the day at Glenelg beach.

They were reported missing by their parents at 7.30pm, five hours after they were meant to return home.

Witnesses told police they had seen the children playing with a tall, blond man in his 30s. The disappearance sparked one of the largest police investigations in the country's history.

A copy of the diary was handed to major crime investigators by South Australian investigative journalist Bryan Littlely, who urged police to seek the original copy from the main diary writer, to whom he had spoken.

"Major crime [investigators] have material which provides evidence people were willing to cover the truth about Munro - evidence dating back to 1965," Littlely said.

In his statement to police as part of the investigation into Munro's abuse, Mr McIntyre alleged his father, now in his late 80s, and Munro, now 72, were involved in the Beaumonts' disappearance.

"The day the Beaumont children disappeared was a shocking day in history that stands out in my memory very clearly," the statement, seen by Fairfax Media, reads.

"I believe Tony and Max were involved in the disappearance of the Beaumont children."

Max McIntyre, a former Telecom worker, was investigated over the disappearance after one of his daughters, Ruth Collins, made the allegations back in 2007.

Police found no evidence he was involved.

Munro, who lived in Glenelg in 1966, was interviewed about the Beaumonts after he was arrested for the abuse of other boys last year. Again, police found no evidence.

In an interview with Littlely in 2015, Mr McIntyre denied his own involvement and pointed the finger at Munro.

Munro's lawyer, Stephen Ey, dismissed the allegations.

"The police who interviewed Munro were satisfied he had nothing to do with it. It's the fanciful ravings of Ruth Collins," Mr Ey said on Wednesday.

Police said the claims about Max McIntyre were extensively investigated and cannot be supported. There was nothing to implicate Munro either, they said.

"Police have undertaken many actions and lines of inquiries in relation to this matter ... there is no evidence at all to support these claims," the police spokeswoman said.

But the diary, Ms Collins and her brother said, supports their assertion that the pair were at Glenelg beach that day.

In a statutory declaration, Mr McIntyre claimed he was meant to go with his father and Munro to Glenelg beach to do some diving, but was told to stay home and they went with other young men.

He said when they came home, both were upset, his father had his hands on his head saying "s---, s---, s---".

Mr McIntyre claims there was sand and blood in Munro's car.

His sister goes even further. She says her father came home wearing a bloodied shirt and, extraordinarily, she claims she saw the children's bodies in the back of the car.

The siblings have demanded that a filled-in well on their father's property outside of Adelaide be dug up, though the well remains undisturbed.

Mr McIntyre was unable to be reached for comment.

The officer in charge of South Australia Police's major crime unit, Detective Superintendent Des Bray, said dozens of people had been named as persons of interest over the years.

These include notorious child killers Derek Percy, Bevan Spencer von Einem - who abducted, tortured and murdered the son of an Adelaide newsreader - and James Ryan O'Neill, who murdered a boy in Tasmania in 1975 and molested others.

Meanwhile, Munro is awaiting sentencing after he pleaded guilty to abusing Andrew McIntyre and another boy between 1965 and and 1983.

The former Adelaide businessman, who was also a Scout leader, was also convicted of the indecent assault of an 11-year-old boy in 1992.

Munro moved to Cambodia to run a bar in the mid to late 2000s before he flew back to Adelaide last year to face the child abuse charges.