"I asked my mum over many, many years why no one asked. She said she wanted to protect me. I said why didn't the police ask, she said she didn't know."

It wasn't until two years ago that two Wollongong police officers who reopened the country's oldest cold case, traveled to Victoria to interview Mr Nash.

"During my affidavit in 2017 at Frankston Police Station, the police officers asked me what I remembered of the day. I said 'everything, it's crystal clear in my eyes'," he recalled this week.

"They couldn't believe no one had ever asked me."

Ricki, his two younger brothers Stephen and Paul and three-year-old Cheryl, had just had a swim on January 12 when they went to the shower block to wash off.

Cheryl had refused to come out of the girl's change room and big brother Ricki, not so keen to enter the female section, had raced off to get his mum.

When they returned less than two minutes later, Cheryl was gone.

When Mr Nash finally got the chance to recount the day, he told detectives in Frankston that he remembered picking up Cheryl and lifting her to the water tap that day.

When asked how he remembered such a detail he said: "I just do."

Soon after, she entered the toilet block and was never seen again. "It was my fault. I shouldn't have left, I should have let one of my brothers go down, I should have been more aware of my surrounds. I will always blame myself, it's now part of my DNA."

It wasn't until the detectives made contact in late 2016, that Mr Nash finally gave up hope of finding his sister alive.

"I've suffered nightmares all of my life, still do," Mr Nash said this week.

"The most devastating thing for me is that while I've lived with the trauma, I've always lived with the dream Cheryl would come knocking on my front door one day.

"I've blamed myself my whole life and that dream was keeping me a bit sane."

When the cold case investigators told him they had good news - they now had a person of interest for the murder - Ricki was shocked, relieved and overwhelmingly horrified.

"I said 'what the f..k. What do you mean she's murdered?'"

The realisation that his little sister would never come home hit him like a tonne of bricks.

But the reality that two police officers had made a breakthrough in the case after all these years - years Ricki spent carrying so much guilt - meant that maybe, finally, his devastated family could get some answers.

The next blow left him an angry, angry man.

A man accused of the abduction and murder, who cannot be named because he was 15 at the time, was scheduled to face trial in the NSW Supreme Court this month. However, in February a judge ruled his detailed 1971 confession to the crime was inadmissible, and the charge was dropped.

In the absence of that interview, there was insufficient evidence for the case to proceed.

The man was released from court that afternoon after almost two years in custody.

Cheryl's accused killer was 17 when he confessed to the murder to two NSW detectives.

Police at the time considered that they did not have enough evidence to charge him, but the re-investigation launched in 2016 led to him being charged.

The confession was legally taken and freely given, but laws were later introduced requiring a parent, other adult or lawyer to be present for child confessions to be admissible in court.

Judge Robert Allan Hulme ruled in February that the newer laws, as they related to police procedure, applied retrospectively.

Prosecutors advised the court within hours that they would not be appealing against the decision.

NSW Attorney-General Mark Speakman sought advice from Crown Advocate David Kell SC after NSW Director of Public Prosecutions Lloyd Babb SC confirmed he would not appeal.

In a statement to the Mercury this week Mr Speakman said: "I have received advice from the Crown Advocate Dr David Kell SC which I am now considering," indicating no further comment would be made "at this stage".

Mr Nash and his family are demanding answers.

In Monday's edition of the Mercury, calls for a monument to remember Cheryl.