To launch an appeal would be cruel and only give the Grimmer family false hope, Mark Speakman said on Friday.

"I have told members of Cheryl's family that I have concluded that any appeal against the Supreme Court's decision will fail and that I will not be bringing any appeal," he said.

"I am sorry for the further distress this will bring to Cheryl's family, who seek justice for her. I am sorry that today I am unable to help end that search."

Cheryl's brothers Ricki Nash and Stephen and Paul Grimmer were made aware of the decision during a meeting with Mr Speakman earlier in the day.

Mr Nash told the Mercury he was gutted by the decision.

"A murderer is allowed to walk our streets a free man unnamed. How is that possible," he said.

Mr Speakman was presented with a document a week ago, outlining why the Grimmers felt the 49-year-old case could be revisited.

But Mr Speakman said he had considered the submissions, applying his own legal training, and sought advice from the Crown Advocate and also held lengthy discussions with the DPP and was confident any appeal would fail.

Cheryl disappeared from Fairy Meadow beach in January 12, 1970. She had been in the shower block at the beach when witnesses claim a man took her and ran off. It is believed that she was strangled to death around an hour after her abduction, in the nearby suburb of Balgownie.

The accused was 17 when he confessed to Cheryl's abduction and murder.

Police did not consider they had enough evidence to charge him, until Wollongong detectives began re-investigating in 2016.

He was arrested and charged in March 2017, making history as one of the oldest Australian cold cases to reach court.

Laws now require a parent, other adult or lawyer to be present for child confessions to be admissible, but this was not the case at the time.

However Judge Robert Allan Hulme found that the new laws applied retrospectively.

It took just 90 minutes for the Director of Public Prosecutions to decide not to appeal a judge's shock decision to exclude the murder confession.

Mr Speakman said he "wished the circumstances of the interview in 1970 were different" but said he could not bring an appeal that was "doomed to fail".

He said he was confident he had left "no stone unturned" but it came as "no surprise" the Grimmer brothers were devastated by his decision.

Unless there was fresh and compelling evidence "this is probably the end of the road for prosecution", he said.