A prisoner died at Whanganui Prison over the weekend.
Prison director Reti Pearse said prison staff found the man when they unlocked his cell on Saturday morning.
Staff members provided immediate assistance, but the prisoner was unable to be revived, Mr Pearse said.
All deaths in custody were referred to the Coroner, and the independent Corrections Inspectorate would also carry out a review.
Police had notified the prisoner's next of kin and support was being provided to staff and other prisoners.
Mr Pearse said he was unable to comment further regarding the identity of the prisoner while the matter was being investigated.