Papers have been filed with the Privy Council in a last ditch attempt to get New Zealand's longest serving prisoner out of prison.

Alfred Vincent was the first person to be sentenced to preventative detention after he was convicted of indecently assaulting seven boys in 1968.

Now 78, he's served 48 years in prison.

His lawyer Tony Ellis has lodged an application with the Privy Council asking for special leave to appeal, given his client has served so long behind bars.

"You do have to have exceptional circumstances for the Privy Council to agree to hear an appeal but I'm hopeful given that he's not gonna get out until he's served 50 years..that's pretty extraordinary."

Ellis says he struggled to find similar cases for his appeal to the Privy Council.

He insisted that other offenders serving jail time for that length of time committed far more horrific crimes, like the Moors murders or Rudolf Hess.

He hopes to have a hearing date in June.