The inmate died Thursday in his cell at Christchurch Men’s Prison.
Prison director Jo Harrex said staff “did everything they could to save him” but were unable to resuscitate him.
Police have been notified and are responsible for notifying the next of kin of the man, Harrex said.
“Our thoughts are with the man’s family and friends. Other men in the unit and staff are supported,” she said.
All deaths in custody are referred to the coroner for investigation and cause of death determination.
An investigation will also be carried out by the independent Corrections Inspectorate.
A source told the Herald that the man was having trouble breathing and had been seeking medical attention for weeks.
Today, however, Corrections said they were unable to answer specific questions from the Herald.
“As the man’s death remains under investigation and the coroner has yet to determine the cause, we are limited in the amount of details we can provide,” Harrex said.
Police did not investigate, a spokeswoman said and referred the investigation to Corrections and the coroner.
A coroner’s spokeswoman confirmed that the man’s death has been reported to them.
“However, as the investigation is at an early stage, no further information is available,” she added.