The 38-year-old owned alternative newspaper NOW which she used to campaign against developers to protect heritage buildings in Potts Point and promote the Builders Labourers Federation controversial green bans during the 1970s.

Ms Nielsen was last seen at the Carousel Cabaret at Kings Cross on the morning of July 4, 1975 - a nightclub run by an associate of notorious underworld crime boss Abe Saffron.

It's widely speculated she was kidnapped and murdered because of her anti-development and anti-corruption campaigns.

A coronial inquiry in 1983 found Ms Nielson, who was also an heiress to the Mark Foy's retail empire, had likely died.

But despite the case generating extraordinary publicity, her remains have never been found.

After more than four decades of heartbreak, her family just wants to give her a proper burial.

Her cousin Francis Foy has appealed for information into her suspected murder, as police announced the reward had been doubled to $1 million.

"Juanita was very much loved by her family and very much missed," Mr Foy said on Monday.

"Her disappearance and the unknown of what happened to her caused incredible pain for her family."

Homicide Squad Commander Detective Superintendent Danny Doherty said the opportunity for police to gather further forensic evidence and witness statements had diminished.

"In turn, it has also become difficult for police to target known persons of interest or associates due to their passing," he said in a statement.

"However, it is our hope that someone in the community may have information about Juanita's disappearance, or the location of her remains."

During initial investigations, police uncovered a plot to kidnap Ms Nielsen four days prior to her last known sighting.

Three men were arrested and charged with conspiracy to kidnap in 1977 and two were subsequently convicted, however no charges have ever been laid over her later disappearance.

Australian Associated Press