Bronson Blessington, condemned to "never be released" from prison for his part in the rape and murder of Janine Balding has applied for clemency under the NSW governor's royal prerogative of mercy.

Blessington was only 14 years old when the crime was committed and the petition is his last avenue to end his indefinite incarceration, found by the United Nations to be "cruel, inhuman and degrading" due to his juvenile status and a failure to consider his prospects for rehabilitation..

He is the youngest person since NSW was a convict settlement to face prison for the term of his natural life, his lawyer, the former NSW MP Peter Breen, said.

In this weekend's Good Weekend, Blessington speaks of his remorse for his crimes, his troubled childhood and the events that led up to one of Sydney's most notorious crimes.

Blessington was in a group of five vagrants – four of them 16 or younger – who in September 1988 abducted Ms Balding at Sutherland railway station and hijacked her vehicle.

The gang drove Ms Balding to Minchinbury, where Blessington and a 16-year-old boy Matthew Elliott sexually assaulted and then drowned the 20-year-old building society teller in a dam near the road.

Blessington and Elliott - along with an older man, 22 year old Stephen "Shorty" Jamieson - received life sentences, with the recommendation from the judge that the crime was "so grave" that the trio were "never to be released".

The mercy petition cites Blessington's youth, his reformed character and the "striking injustice" of being the subject of retrospective legislation from the NSW government that removed his appeal rights.

Under the laws at the time, the judge's "never to be released" recommendation carried no legal force. Prisoners handed life sentences has the right after eight years to apply to have the penalty commuted on the grounds of good behaviour.

The petition also highlights Blessington's "exemplary" disciplinary record in prison. Along with other serious offenders, Blessington's security classification was downgraded to medium last year but was lifted back to maximum following a tabloid media campaign.

"In the normal course of sentencing by the courts – as opposed to intervention by the state parliament – Mr Blessington would still not be in prison," the petition said.

"[The] natural life sentence contravened the principle that a prisoner should be sentenced according to the sentencing regime that applied at the time of his crimes," the petition says.

Blessington can only be released if he is "imminent danger of dying" or "so incapacitated" that he no longer has the "physical ability to do harm to any person".

"The only realistic opportunity for the prisoner Blessington to be released on parole is for the NSW Governor to exercise the royal prerogative of mercy. And the Pope Francis Year of Mercy would be a good time to do it," said Mr Breen.

"Spending the rest of you life behind bars without any prospect of parole would send most people insane."

The Balding family strongly opposes the release of Blessington, saying his youth was irrelevant in light of the horrific nature of the crime. Ms Balding's brother David said Blessington was where he belonged.

"He knew exactly what he was doing".

The NSW governor will act on the advice of the NSW attorney general Gabrielle Upton. A spokesperson for Ms Upton said Blessington's application was being considered "according to the usual protocol".

Mr Breen, who entered parliament for the Reform the Legal System Party, and was part of the successful campaign to establish the innocence of Roseanne Catt, wrongly imprisoned for the attempted murder of her former husband. .