Cheryl Grimmer was kidnapped on January 12, 1970, outside a shower block at Fairy Meadow Beach in Wollongong where she had been spending the day with her mother and three brothers.

Relatives are hopeful of a breakthrough in the cold case as the community on Sunday rallied about them on the anniversary of her disappearance.

A man had been due to face trial last year but a key piece of evidence - an admission made during a police interview in 1971 - was ruled inadmissible by the Supreme Court.

In the hope of turning up new evidence, the NSW Police homicide squad on Sunday announced a $1 million state government reward for information leading to an arrest and conviction.

"We need justice," Cheryl's brother Paul said.

"But it will come. It's taken a while, but it will come,"

Despite extensive inquiries over the last five decades, Cheryl's body was never found.

A 2011 coronial inquest found that Cheryl had died but the manner of her death remained undetermined, and recommended the case be referred for further investigation.

The case was re-opened and investigated by Wollongong Police District eight years ago under Strike Force Wessel. It has been referred to the Unsolved Homicide Unit for potential further re-investigation.

Homicide squad commander Daniel Doherty said detectives would welcome any information from the community.

"Witnesses at the time reported seeing an unknown male carrying Cheryl towards the car park 50 years ago today but there has been no trace of her ever since," Det Supt Doherty said.

On Sunday members of the Grimmer family and the community walked from Balgownie to Fairy Meadow Beach, tracing where it's believed her body was dumped down to where she was taken.

The Wollongong City Council also unveiled a plaque commemorating Cheryl's life.

"It just goes to show, it's not just our family and friends, it's people of the Illawarra," Mr Grimmer said.

Australian Associated Press