An 81-year-old with the "doubtful distinction" of being the longest-serving prisoner in New Zealand has again been denied parole.

Alfred Thomas Vincent was one of the first people to be sentenced to preventive detention in 1968 after he was convicted of indecently assaulting five boys aged 12-14.

The child sex offender has spent more than 50 years in prison.

The only time Vincent has spent outside prison was during day passes and weekend leaves in the early 80s - which were revoked when he was caught talking to young boys.

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The Christchurch man is suffering from health issues to the extent there was no possibility for the Parole Board to "engage with him directly".

READ MORE: Who has spent the most time behind bars?

In the decision released today, Parole Board panel convenor Judge Eddie Paul said under "trying circumstances" Vincent's lawyer had made a "tentative submission" for the 81-year-old's release.

"This Board is concerned that really no formal release proposal to [withheld] has been presented," Paul said.

"We would expect as a minimum there would be a multidisciplinary meeting including his GP in the prison, [withheld], intending to really set out and determine this man's needs."

Certainly, any proposed hospital care provider would need to attend that meeting so they are "fully aware of the various features" Vincent presents, Paul said.

"It is very clear from the reports we have that he is continuing to focus on inappropriate sexualised behaviour," he said.

"That presents risks for not only himself but any other patients of any facility he should be referred to."

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Paul invited Corrections and other professionals to prepare a "robust release proposal" addressing all the documented concerns for Vincent's next hearing in August.

"Obviously, he is not a candidate for parole today."