A bid by New Zealand's longest serving prisoner to overturn his sentence imposed nearly half a century ago has been rejected by Privy Council in Britain.

Pensioner Alfred Vincent, 78, has been behind bars since he was sentenced to preventive detention in September 1968 for seven indecent assaults against five boys, following two earlier jail stints in 1964 and 1966 for performing indecencies on boys.

The only known photograph of New Zealand's longest serving prisoner, Alfred Thomas Vincent, now aged 74. He was a smiling 12-year-old schoolboy in his Standard 2 class photo at Kaiapoi School in 1949 and was 25 when first convicted of sex offending against children, but claimed he started at age 15.

In a written judgment on April 12, the Privy Council refused Vincent's application for leave to appeal a November 1968 Court of Appeal decision "because there is no risk that a serious miscarriage of justice has occurred in the case".

Vincent's lawyer, Dr Tony Ellis, said on Wednesday he would seek a judicial review of Vincent's sentence and would consider complaining to the United Nations Working Group on Arbitrary Detention.

He was disappointed at the council's rejection and believed the length of Vincent's detention was "inhumane".

"I can't say I'm surprised about the Privy Council's decision because it is trying to ascertain what happened 48 years ago."

The Supreme Court became New Zealand's highest appeal court on January 1 in 2004, replacing the Privy Council, but offenders whose cases were decided prior to that date still had the right to appeal to the British law lords.

Court of Appeal records of Vincent's failed attempt to overturn his preventive detention sentence in November 1968 stated Vincent's then-lawyer "had the unenviable task" of trying to persuade the judges to substitute it for a finite sentence.

"We have listened to all he has said, but we are quite satisfied that the case was so bad that there is no question that the appellant has justified for a sentence of preventive detention, the sentence imposed on him by McArthur J," the 1968 court decision stated.

In 1968, an indecent assault conviction gained a seven-year maximum jail term. As a result, Vincent, who grew up in Kaiapoi in Canterbury, became eligible for parole 41 years ago and had appeared in front of the Parole Board almost every year since.

Last October, the board issued a three-year postponement order, meaning he must wait until 2018 for his next chance for release from Rolleston Prison in Canterbury, where he had spent most of his incarceration.

A Parole Board member heads into a parole hearing for Alfred Vincent at Rolleston Prison, near Canterbury, last August. DEIDRE MUSSEN/FAIRFAX NZ.

Vincent had become increasingly frail in recent years, had an artificial eye, wore hearing aids in both ears and suffered heart problems, the board was told.

However, its decision noted he remained a risk to public safety despite his age.

"He is New Zealand's longest serving prisoner and sad as it may be, he remains a significant high risk sexual offender who, by his own admission... to the police, believed he would reoffend on release and pose a threat to young males."

The board said it would be "inhumane" to free Vincent because he was unable to function properly in the community after such a lengthy incarceration.

ALFRED VINCENT'S CRIMINAL PAST

* October 1956: One charge of theft. Two years' probation.

* July 1963: One charge of performing an indecency with an 11-year-old. Eighteen months' probation.

* January 1964: Three charges of performing indecencies with boys aged 8-11. Jailed for a year.

* January 1966: Two charges of performing indecencies with a boy aged under 16. Jailed for 18 months.

* February 1968: Fined $60 on three dishonesty charges.

* September 1968: Seven charges of indecent assaults on five boys aged 12-14. Sentenced to preventive detention.

* November 1968: Court of Appeal rejects his appeal for a finite sentence rather than the indefinite preventive detention.

- Deidre Mussen/Stuff