A man has died at Christchurch Men's Prison.
The Corrections Department said the prisoner was found unresponsive in his cell yesterday.
He was given medical attention, but died.
Police said they were called to the prison about 8am yesterday.
A spokeswoman said they believe the man died of a medical event.
As the death was in custody, it will be subject to a Corrections Inspectorate review and a Coroner's inquest.
The department won't comment further.