A man died at Springhill Corrections Facility in Waikato on Wednesday.
Prison Director Scott Walker said staff made every effort to save him but were unsuccessful.
There is no indication the man's death is suspicious and police are notifying his next of kin.
"Our thoughts are with the man’s family and friends. Other men in the unit and staff are being provided with support," Walker said.
All deaths in custody are referred to the Coroner for investigation and determination of cause of death. An investigation by the independent Corrections Inspectorate will also be carried out.
"As the man’s death remains subject to investigation, and the Coroner is yet to determine the cause, we are limited in the amount of detail that we are able to provide," Walker said.