A serial paedophile will have spent 50 years behind bars before he's eligible for parole again.

The Parole Board has imposed a three year postponement order on Alfred Vincent who was jailed in 1968 for prolific sexual offending against young boys.

The 77-year-old has now been in jail for 48 years.

The Parole Board says if it was to release Alfred Vincent, it's highly unlikely he'd be able to function in the modern world.

The board says, while Vincent's sad history suggests he never be able to satisfy the board he'll be suitable for release, it can never be the case that all hope is lost.

It says given Vincent has been in jail for just about all of his adult life, he would require considerable supervision and support if released on parole.

The Parole Board now has the power to impose postponement orders of up to five years.