Mr Cowdery – currently a member of the independent body that advises NSW Attorney-General Gabrielle Upton on sentencing – said the imprisonment of a juvenile for an indefinite term was "objectionable" and motivated by "pure revenge".

Moreover, the retrospective laws from the NSW parliament that stripped Blessington of his appeal rights and any prospect of parole until he is at death's door were "deplorable", he said.

Blessington was 14 years old when he took part in the abduction, rape and murder of Janine Balding at Sutherland in September 1988, one of the state's most notorious crimes.

In an interview with Fairfax Media's Good Weekend magazine, Blessington, now 42, described remorse for his crime and outlined his deeply troubled upbringing and the events that led up to a brutal killing.

Citing his rehabilitation and "exemplary" disciplinary record in prison, Blessington's lawyer Peter Breen has petitioned the NSW Governor to be granted mercy, the only avenue left for the prisoner to be released.

The governor will make his decision on the advice of Ms Upton.

"This is a case for the exercise of the prerogative of mercy," Mr Cowdery said, adding he was speaking in a personal capacity and not for the NSW Sentencing Council..

"Just putting someone in a box and holding them until they die is objectionable, particularly when you do this to someone in their teens," he said. "I don't think there is any rational justification for it at all. It's revenge pure and simple."

While retribution was one principle of the criminal justice system, Mr Cowdery said rehabilitation was another.

"Families of the victims have a legitimate say but they don't have – and they shouldn't have – the power to dictate what is going to happen," he said. "Their views must be taken into account but they must be weighed against the other considerations."

Ms Balding's family has been adamantly opposed to the release of Blessington, saying the crime was so abhorrent he deserves never to be released regardless of his age or rehabilitation.

They have said the release of Blessington would deeply distress them.

"I'll show him the same mercy he showed my sister," Janine's brother David Balding told Wagga Wagga's Daily Advertiser this week. "He should not be out of jail until he is dead."

Member for Wagga Wagga Daryl Maguire told the paper he would urge Ms Upton to deny the mercy petition and was confident it would fail due to ongoing public outrage over the infamous crime.

"The Balding family does not deserve this from a piece of garbage that took the life of a beautiful girl," Mr Maguire said. "He had a choice, he made a bad decision, now he has to pay for it."

Along with some states in the United States, Australia – notably in NSW – is the only other country in the western world known to incarcerate juveniles for the term of their natural life.

It is understood the NSW Sentencing Council has written a report urging the NSW attorney-general to remedy the situation, which the United Nations found breached international treaties that Australia has ratified.

Ms Upton declined to answer questions but a statement from her department confirmed that children still could be sentenced to life in prison without parole in NSW.

The Law Council of NSW on Friday backed Mr Cowdery's call of mercy for Blessington.

The head of its criminal law committee, Pauline Wright, said the three sets of retrospective laws passed by the Carr government to "cement in" Blessington, Elliott and nine other serious offenders in prison ran contrary to core principles of the justice system.

As well as denying them the chance of rehabilitation, it left them exposed to a sentencing regime that didn't exist when they committed the crime.

Under the laws at the time of the crime, Blessington had the right to apply for a fixed term after eight years in prison.

Now, he can't be paroled unless he is about to die or so incapacitated he could not commit a crime.

For Blessington, that means he easily could spend another 40 or 50 years in prison, on top of the 28 he has already served. Blessington has always been held in maximum security, meaning he is allowed out of his cell for only a few hours a day and cannot work in prison workshops.

His security classification was recommended to be downgraded to medium last year but a tabloid media campaign saw that decision reversed.

Victims rights groups were contacted but did not return calls before deadline.