Robbed, abducted, sexually assaulted and murdered in her home suburb of Blacktown, the Sydney Hospital nurse's senseless death became "a defining part of our nation's criminal history".

On Monday morning, nearly three decades on, a crowd of 300 gathered in Minchinbury to remember the 26-year-old whose brutal end at the hands of five men came to shape state responses to victim support and violence towards women.

"The offenders' intentions were to destroy and kill. Our intentions are the opposite," said retired Chief Inspector Gary Raymond, who worked on the case and led the service, the first public memorial for the victim.

Honouring his promise to Ms Cobby's father, the late Garry Lynch, to keep his daughter's memory alive, the service was the first since the death of her mother, Grace Lynch, last year.

Ms Cobby was abducted on February 2, 1986, while walking home from Blacktown train station. She was taken to nearby Prospect, where she was raped and murdered.

Five men were sentenced to life imprisonment for her murder, never to be released.

At Ms Cobby's memorial on Monday, a candle-lighting ceremony, a butterfly ceremony and a rendition of the Eurythmics' Miracle of Love ensured the mood was focused on the triumph of good over evil and, in the spirit of her parents, in community strength.

It was the tireless work of Mr and Mrs Lynch that saw the eventual foundation of the Homicide Victim's Support Group.

Julia Sheppard, author of Someone Else's Daughter, the life and death of Anita Cobby, said the murder prompted a public outcry unlike any other.

"She's become a defining part of our nation's criminal history … It united the community in a way that hadn't been seen in modern times," she said.

In the congregation were leader of the opposition Luke Foley, his predecessor John Robertson and scores of members of the NSW police force, including those from the team who worked on the case at the time. Also in the crowd were Peter Simpson, father of murdered nine-year-old, Ebony, and the parents of murdered policeman Bryson Anderson.

Kathryn Szyszka, Ms Cobby's younger sister, told the crowd that it was not a day to dwell on negativity but, instead, to highlight the plight of the no violence towards women and White Ribbon campaigns.

Her voice breaking as she spoke of her sister's unanswered cries for help all those years ago, she echoed her parent's words: "That must never happen again."

She told Fairfax Media that the day was "overwhelming but beautiful" and that the support was as vital as ever.

"It's such an important thing, it's very apparent the good things that are happening and continue to happen to her memory - it's what keeps us going."

Federal MP for Greenway Michelle Rowland was a teenager at the time and used Blacktown station, where Ms Cobby was abducted, twice daily on her journeys to and from school.

"I remember it vividly - it was so foreign to us, we were scared," she said after the service. "It was an abomination. These are ordinary people - we need to remember this is what stopping violence towards women is about."