An Otago Corrections Facility prisoner died in Dunedin Hospital on Wednesday night.
Acting prison manager Vicky Frost confirmed a 53-year-old prisoner collapsed in his cell at 4.30pm.
The man was given immediate medical attention, and later taken to hospital by the Otago Regional Rescue Helicopter.
He died later that night.
"It appears the death was from natural causes. However, the coroner will determine the exact cause.
"Our thoughts go out to the prisoner's family at this time,'' she said.
All deaths in custody are investigated by the coroner, to determine the cause of death, and the Corrections Inspectorate, which is monitored by the Office of the Ombudsmen.
The department was unable to make any further comment until all investigations were complete and the coroner had released inquest findings.
