There have been many theories about the Bogle-Chandler deaths on New Year's Day 1963, including murder, assassination and poisoning, but in the six decades since nobody has ever come forward as an eyewitness.

Now a retired Canberra psychologist has told author Peter Butt that one of the young women told him that she had gone to the riverbank with her girlfriend after attending a New Year's Eve party.

On leaving, the woman realised she had lost her purse. As they retraced their steps, they spied another couple arrive. They then watched from the bushes as Bogle and Chandler engaged in sexual activity.

In his book, Dr Bogle and Mrs Chandler: The Confession, Butt quotes the psychologist's recollection of the young woman's eyewitness account in a conversation in January or February 1965, two years after the deaths.

"At some point, the woman said loudly, 'Why have you stopped?'. He said nothing and she asked him to 'Keep on going'. Then suddenly the woman grabbed her throat and made a strangling noise and got up and staggered off," the psychologist recalled.

"After a few minutes, he rolled over or moved some short distance and my memory suggests she said he moved away from the water and up the slope.

"They waited a little while till there was no noise to look for the purse.

"They smelled some rotten egg gas, but it seemed to go away as they moved closer to the edge of the water. As they searched, they discussed why the couple had acted so strangely and just assumed they'd been on drugs or something."

The Bogle-Chandler case riveted Australia for years. It had everything for a country drifting towards the end of the straight-laced Menzies era: the hint of murder, illicit suburban sex, drugs, political assassination, espionage, nuclear secrets, bohemian immorality and the inability of police to find a culprit.

Australia was then a very different place. Homosexuality was a crime; equal pay a decade away; women were mainly regarded as a function of their husbands; with Beatlemania and the Baby Boomer economy a year ahead, the young were seen and not heard; Australians were not allowed to read D.H. Lawrence's four-letter word classic on English inter-class sex, Lady Chatterley's Lover; and it was just two months since the Cuban missile crisis took the world to the brink nuclear war.

Then the unclothed bodies of Dr Gilbert Bogle, 38, and Mrs Margaret Chandler, 28, were found and suddenly the seamy side of Menzies suburbia was exposed full frontal.

Bogle, New Zealand-born and a former Rhodes Scholar was a physicist with the Commonwealth Scientific and Industrial Research Organisation and married with four children. He was supposedly working on futuristic weaponry. He also had a penchant for hitting on women and taking them to parks.

Chandler was a housewife with two children. Her womanising husband, Geoffrey, worked at the CSIRO with Bogle.

The three attended a New Year's Eve party at a CSIRO colleague's place in Waratah Street, Chatswood, on Sydney's north shore. Vivienne Bogle stayed home with the children in Turramurra.

Geoffrey Chandler left soon after midnight to meet his mistress Pam Logan at a party in Balmain that featured many members of the Sydney Push, went back to her Camperdown flat for an hour and then returned to Chatswood where he told police he asked Bogle to look after his wife, assuring her he would pick up their children from her parents' place.

Bogle and Chandler left Chatswood after 4am and drove to the well-known "lovers lane" near Fullers Bridge. His body was first spotted by a teenage boy about 8.30am. Police found Mrs Chandler's body nearby on the riverbed.

In 2006, Butt's Logie award-winning ABC documentary Who Killed Dr Bogle and Mrs Chandler? claimed their unsolved deaths may have been caused by accidental hydrogen sulphide poisoning from industrial waste that had concentrated in the bottom mud of the Lane Cove River. The new book, a revised edition released in August, details the psychologist's story.

Butt said in 2012 he was contacted by the retired psychologist, who was keen to pass on information about the deaths. Butt has not identified the psychologist.

The psychologist told him he had saved a young woman from being sexually assaulted by a group of drunken young men in Haig Park, Canberra, about 5am one summer day in 1965.

She said she did not wish to go to the police or hospital, but noticing a St Christopher medal round her neck, he asked if she was a Catholic and would she like to be taken to a nun he knew at a convent in Manuka.

The young woman then broke down. She said she was having nightmares due to something she had witnessed two years earlier at Lane Cove.

She told the psychologist she and her girlfriend – who was dressed as a man – had watched the last moments of Bogle and Chandler and had been too scared to come forward.

(The woman dressed as a man scenario feeds into evidence given by a one-armed voyeur, Raymond Challis, a police prime suspect who spoke of seeing a broad-shouldered man with "blonde longish hair at the back" jump out of the bushes when he was creeping around the riverbank at the time of the deaths.)

"She said that was because they were 'good Catholics'… they were worried what would happen to them and their families if their friends found out what they were doing together," the psychologist told Butt.

The women broke up and the psychologist was quoted saying he feared the young woman was suicidal and "exposure could have pushed her over the edge".

He told Butt the woman never told him her name but he recalled she had an aunt who was a nun and an uncle who was a priest.

The psychologist also said he occasionally saw the woman in the car park of the Department of Trade building in Barton, ACT. They'd wave, but never spoke again.

Butt said the psychologist has signed a statutory declaration detailing the story, which contains details of the "crime scene" that could only have come from someone at the river that morning.

The State Coroner has forwarded the statutory declaration to NSW Police and the Unsolved Homicide Team had advised Butt to make the story public.

"Both women would now be in their mid-70s. There is every chance that they are still alive," Butt said. "Their testimony could trigger a new inquest and hopefully bring closure to the case, just over half a century after the event."