Cheryl was just three years old when she was kidnapped outside the change sheds on January 12, 1970 after a day at the beach with her mother and three brothers Stephen, Ricki and Paul.

Despite extensive searches at the time and over the past five decades, police have found no trace of Cheryl.

In September 2012, the NSW government offered a $100,000 reward for information on her abduction and suspected murder.

A review of the cold case by Wollongong detectives led police to the arrest of a Victorian man in March 2017. He was charged with murder however the case against him was dropped last year after key evidence was ruled inadmissible for use in court.

In announcing the increased reward on Sunday, NSW Police homicide squad commander, Detective Superintendent Danny Doherty, said Cheryl's family deserved answers.

"Now is the time to come forward, now is the time not to hold back," he said.

"If you've been holding a secret, and you think you might be protecting someone or you have information, please come forward.

"It's important to the family, its important to the community."

Cheryl's brother Ricki, the last family member to see her alive, said the family prayed every day for answers.

"We know the answers are in front of us," he said.

"We believe somebody has the answers that can get this person, this evilness, to come out of the shadows, to come forward."

Mr Nash said as much as he and Cheryl's extended family feared hearing details of her death, they needed it for closure.

"If we can get into a courtroom and hear what happened to Cheryl that day....as much as I never want to hear it, I have to hear it," he said.

"No words can explain what we've been through. No family should have to go through this. We want some closure."

Anyone with information that may assist Strike Force Wessel investigators is urged to contact Crime Stoppers: 1800 333 000 or https://nsw.crimestoppers.com.au. Information is treated in strict confidence