A prisoner has died at Mt Eden prison this week.
Acting prison director Dennis Goodin confirmed the man died on Monday morning.
"Medical assistance was provided by on site staff and emergency services were called, however the prisoner was pronounced dead at the scene."
Mr Goodin said the death was an apparent suicide.
The Herald understands the man was in solitary confinement, in a cell known by inmates as "the pound", at the time of his death.
The death comes after Corrections stepped in to run privately owned Mt Eden Corrections Facility following claims of organised "fight clubs" at the Serco facility.
Labour's Corrections spokesman Kelvin Davis told the Herald that any apparent suicide in prison was "unacceptable".
"It's just terrible. I'd like to know the circumstances around it, especially his monitoring," Mr Davis said.
"If he was in segregation, he should have been watched often."
Yesterday, Mr Davis met the parents of Northland man Anthony Stillwell, who died in an apparent suicide at Paremoremo in September last year.
He said the family still had unanswered questions six months later.
"It's a tragedy again, and there's some family now that's having to go through what the Stillwell family had to go through."
The Department of Corrections said it was committed to preventing unnatural deaths, adding that prisoners had higher rates of mental illness than the general population.
"Corrections has been working to reduce the suicide rate," Mr Goodin said.
"Initiatives include mental health training for frontline staff, introducing a mental health screening tool, improving referral processes for forensic care, and providing a more therapeutic environment in the at risk units. The department's processes ensure many prisoners are prevented from self-harming or attempting suicide."
Mr Goodin said Corrections staff had saved the lives of about 100 prisoners over the last five financial years.
"These prisoners were involved in self harm incidents where the individual would have been unlikely to survive without staff intervention."
Monday's death has been referred to the coroner.
Unnatural deaths in custody at all NZ prisons:
• 2015/16 so far as at February 29, 2016 - 8
• 2014/15 - 8
• 2013/14 - 3
• 2012/13 - 3
• 2011/12 - 5
• 2010/11 - 12
(Source: Corrections. Unnatural deaths include suicide, homicide, death by drug overdose, and death by accident)