Four decades later and her family are still holding out hope for any information about her whereabouts.
Her granddaughter, Natalie Jackson has appealed to anyone with any information to come forward.
"We are really hopeful the passing of time means someone may feel more comfortable to come forward to help us uncover what happened to our grandmother," she said on Monday.
"To everyone else my grandmother is a missing person, but to us she is just really missed."
The woman with grey hair and blue eyes who lived with a number of medical conditions was on her way to Leichhardt swimming pool in Sydney's inner west in April 1983.
Despite efforts by her family and the police, she was never seen or heard from again.
In 2008, the NSW Deputy State Coroner declared Heather was deceased.
No finding was made regarding her manner or cause of death.
Heather is believed to have had a partner named Ray, who her family believe was an interstate truck driver who drove the route between Sydney and Melbourne.
NSW Police Minister Yasmin Catley is highlighting Marks' case as part of Missing Persons Week, an annual national campaign running from August 3 to August 9.
The 2025 theme "Forever Loved" focuses on acknowledging the experiences of families of missing persons and the impact it can have on their lives.
"Behind every missing person is a family left waiting for answers," she said.
"No family should be left in the dark for four decades."
Assistant Police Commissioner Scott Cook said about 10,000 people go missing in NSW every year.
16°C