An inmate is dead after an incident at Hawkes Bay Regional Prison yesterday.
The Herald understands the death was a result of a "serious incident" and staff at the prison near Hastings were shaken.
The Department of Corrections confirmed the death, but could not elaborate on the specifics including the inmates name and age.
Acting prison director Nephi Hall said custodial and medical staff provided immediate assistance to the inmate but he could not be revived.
"Police are investigating on behalf of the Coroner, who will determine the cause of the man's death," Hall said.
"The Corrections Inspectorate will also carry out a review."
Hall said support was being provided to staff and prisoners.
"We are unable to provide further comment while the matter is subject to investigation."
The prison opened in 1989 and can house a total of 682 minimum to high security male inmates.
In the 2015/16 year there were 11 "unnatural" deaths recorded in New Zealand prisons and 26 cases of "self-harm threat to life incidents".
The figures increased from 8 deaths and 4 incidents in 2014/5, 3 deaths and 17 incidents in 2013/14, 3 deaths and 7 incidents in 2012/13 and 5 deaths and 6 incidents in 2011/12.
"The number of unnatural deaths in prisons has increased since last year, as has the number of self-harm threat to life incidents," Corrections stated in its last annual report released in October.
The most common cause of unnatural death in prison is suicide.
Unnatural deaths also include homicides, accidental deaths and drug overdoses.
"Suicide is a serious health and social issue for New Zealand as well as for the offenders that Corrections has responsibility for," the report stated.
New Zealand has one of the highest rates of suicide among developed countries.
Statistics New Zealand measure the rates of suicide deaths per 100,000 every four years and in 2012 the revealed that the number of men who committed suicide was 18.1 per 100,000.
Furthermore, the number of Mäori male who committed suicide was 25.6 per 100,000.
"The statistics are significant for Corrections as around 90 per cent of our offenders in prison are male and just over half of our prison population identify as Mäori," the 2015/16 annual report said.
"As a result of Corrections awareness around mental health and suicide, clear guidelines, good practice and capable staff, unnatural deaths were avoided in 70 per cent of cases where staff intervened in instances of life threatening self-harm by prisoners.
"It can be difficult to prevent someone from harming themselves if they are determined to do so."
After an inmate has died of unnatural causes Corrections undertake a formal "Death in Custody Review".
The review is carried out by the Chief Inspector of the Corrections Inspectorate - which is monitored by the Ombudsmen.
The Coroner then undertakes an inquest.
Corrections said both of these outcomes can take 6-12 months depending on the complexity of the incident.