Nielsen disappeared from Kings Cross on the morning of July 4, 1975. She published the independent newspaper NOW, which campaigned against the demolition of historic terraces in her street in Potts Point, but also supported the broader union green bans, which sought to preserve older parts of Sydney and protect them from redevelopment.

Her body has never been found and while two men were convicted of conspiring to kidnap her, no one was ever brought to justice for her presumed death.

Rees wrote about the case in his 2004 book Killing Juanita, forcing a federal parliamentary inquiry and a secret reopening of the case by NSW Police. One of the bombshell findings he made was that the highest level of the police force thwarted the investigation into her disappearance.

Last week, a major Australian film director (who we aren't allowed to name yet) visited Rees at his Narrabundah home to interview him for a documentary being made about Nielsen. It's expected to air on the ABC later this year.

Rees said the filmmaker wanted to create a whole picture of Nielsen and was keen to talk with her friends for the documentary.

While Nielsen went to school at Ravenswood School for Girls in Gordon on Sydney's upper north shore, Rees believed at least some of her former classmates were living in Canberra.

"The National Museum set up a display as part of contemporary history on the story of Juanita Nielsen and they asked me to give a talk. That was September 2004, I remember," Rees said on Friday.

"And a group of women came along, seated up the back. I went to talk to them later and they said they had gone to school with Juanita and they were all Canberrans."

Nielsen would have been 84 on April 22. Rees said the filmmaker was keen to connect with the school friends and ask them about their recollections of her.

"I'd love to talk to them again myself," he said, adding he wanted to update Killing Juanita.

An heiress to the Mark Foy's department store fortune, Nielsen was lured to the Carousel Cabaret nightclub in Kings Cross on the promise of a meeting about advertising in her newspaper.

She was never seen again.

Rees has lived with the mystery ever since.

"It's a story that's never left me. I've been working on it all the time," he said.

Nielsen did not have children but her surviving cousins and great-nieces still want closure. And the documentary might just help deliver that.

"The family is hoping this process just might shake some information. Somebody who might have heard something might come forward," he said.

And Nielsen's legacy lives on. While some terraces were ultimately demolished, the essential streetscape was preserved, as was her home at 202 Victoria Street, Potts Points.