A Parole Board member heads into a hearing for Alfred Vincent, New Zealand's longest-serving inmate, at Rolleston Prison, near Canterbury in August.

No freedom is in sight for New Zealand's longest serving inmate, who will have clocked 50 years in jail before he is next allowed to seek parole.

The Parole Board has issued a three-year postponement order to pensioner prisoner Alfred Vincent, 77, who has been locked up since being sentenced to preventive detention in 1968 for seven indecent assault charges against boys.

Vincent's Auckland-based lawyer, Dr Tony Ellis, said the board had given Vincent a three-year postponement order, which was less than the five-year maximum allowed under newly-introduced parole laws.

"It rounds [his incarceration] up to 50 years before he can possibly get out.

"I think it's inhumane to sentence anyone to 50 years in prison for indecent assault," said Ellis, a human rights lawyer.

The board was yet to publicly release its reserved decision after a postponement hearing was held via video link on October 5 with Vincent, who was in Rolleston Prison in Canterbury, where he had spent most of his lengthy incarceration.

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Last year, Vincent and a supporter contacted Ellis for help after seeing media coverage of a case he took to the United Nations Working Party on Arbitrary Detention, which involved an intellectually disabled inmate locked up for decades in prison and mental health institutions.

Ellis hoped to lodge Vincent's case with the same UN Working Party by the end of the year and also planned to seek a judicial review of the board's decision to decline him parole.

Vincent's detention was "hugely disproportionate" to the offence, he said.

Seven convictions for indecent assaults on five boys aged 12 to 14 had a maximum jail sentence of seven years, but Vincent got preventive detention in 1968 because of his previous child sex convictions.

Vincent, originally from Kaiapoi in Canterbury, has a glass eye, wears two hearing aids, and has serious heart condition.

It was "laughable" that he was still considered a risk, Ellis said.

In September, the board said in its decision to decline parole for Vincent that he still posed a high risk of sexual re-offending.

"Despite his advanced years, he is said to exhibit a strong and persistent pattern of highly sexualised interest and behaviour. If anything, he has become more disinhibited. But he is not amenable to treatment," its written decision said.

"All agree that he would need an 'extremely robust' release plan to remain offence free in the community. Effectively he would need 24 hour supervision and care."