The death of inmate at Auckland South Corrections Facility, also known as Wiri Prison, has been deemed unsuspicious by police.
The prison, operated by Serco, is located in the south Auckland suburb of Wiri. It houses prisoners in minimum to high-security areas.
Serco director Mike Inglis said a 62-year-old prisoner was found dead in the self-care accommodation unit on Wednesday morning.
While police earlier said they were treating the death as unexplained, they have since told Newshub they "do not believe the death is suspicious".
The matter has been referred to the Coroner, and police are still awaiting the results of a post-mortem examination.
Police and an ambulance were immediately notified of the death and next of kin has been notified. The Corrections Inspectorate will investigate the death.
Counselling and other support has been offered to other prisoners and staff.