Alfred Vincent has spent over 50 years behind the wire.

An increasingly frail 81-year-old inmate who has been in jail for over 50 years will spend more time behind bars.

Alfred Thomas Vincent, New Zealand's longest serving inmate, was sentenced to preventive detention in 1968 for indecent assaults on five boys after two previous jail stints for similar offending. He is believed to have sexually abused over 100 boys.

Vincent is housed in Rimutaka Prison's high dependency unit due to health conditions that include mental impairment, understood to be dementia, and a heart condition. He has hearing aids in both ears and a glass eye.

In 2016 the Privy Council rejected a bid to have him released.

In a decision in November, the Parole Board said Vincent continued to decline in both health and cognitive functioning and continued to display inappropriate behaviour.

The Board noted a psychologist's recommendation that Vincent's risk could be managed by an unnamed residential facility.

"We are not certain that we share that confidence," the Board said.

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Vincent's lawyer did not seek parole and acknowledged limited prospects for his safe accommodation existed.

She suggested Vincent might be eligible for compassionate release but the Board said it was not satisfied Vincent met the legal test.

ROBERT KITCHIN/STUFF Alfred Vincent is held in Rimutaka Prison's high dependency unit.

Any compassionate release would require the availability of suitable accommodation for Vincent, it said.

"The risk remains undue at this time in the absence of a clear release plan and proposal for Mr Vincent directed at both his care and the safety of the community including those other people who may be residing with him in any facility and their visitors," it said.

The Board will see Vincent, who was brought up in Christchurch, in May next year.