Corrections' northern regional commissioner Jeanette Burns said staff gave Stillwell medical attention but he was pronounced dead at the scene.
The death would be subject to a Coroner's inquest and corrections would also investigate the death, she said.
"Any death that occurs in custody is a tragedy and our thoughts are with his loved ones during this time."
Because the death was the subject of a Coroner's Inquest, Corrections would not comment further.
Labour corrections spokesman Kelvin Davis said no death in custody was acceptable and more needed to be done to help troubled inmates.
"We've got to do more to make sure that people that are in there and need help that they deserve are getting it - to that when the are reintegrated back into society they can be reintegrated," he said.
The death comes only a month after Liberty Charles Baker, 63, was found dead in his cell at Mt Eden prison.
At the time, Ms Burns said inmates were often suffering from extremely poor mental health when they arrived in prison and the care they received in prison often far exceeded what they were accessing in the community.
"Despite the significant efforts we make, it is incredibly difficult to prevent the actions of someone who is determined to hurt themselves."
She said this was a common issue worldwide where mental health disorders and illness is up to five times more prevalent in prisons than in the community.
"In saying this, our staff have saved the lives of approximately 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."
Chief executive of The Howard League for Penal Reform, Mike Williams, said conditions at Paremoremo were "Dickensian" and if Stillwell had been at the prison for only four days it was likely the full horror of a 12-year sentence had hit him hard.
Mr Williams said mental health care offered by Corrections had "improved" over the past few years, and if Stillwell was on hourly watch then it was likely there had been concerns about him.
Corrections data released to the Herald shows that between June 30, 2009 and May 31, 2015 there were three "unnatural deaths" at Auckland Prison in Paremoremo.
Unnatural deaths include suicide, homicide, death by drug overdose, death by accident, and where there is sufficient evidence to suggest that the event was most likely not due to natural causes.
Over that almost six-year same period, Corrections staff intervened in 11 attempted suicides at Paremoremo.
Between 2009 and May this year there were 115 attempted suicides in New Zealand prisons.
Nationwide, there were 82 unnatural deaths between 2009 and May this year, eight of which occurred between June 2014 and May 2015.
Corrections conducts health screenings when offenders enter and are transferred between prisons.
A mental health screening tool and suicide prevention framework has also been developed.
Unnatural deaths in custody at all New Zealand prisons:
2014/15: 8
2013/14: 3
2012/13: 3
2011/12: 5
2010/11: 12
2009/10: 6
Unnatural deaths include suicide, homicide, death by drug overdose, and death by accident, and where there is sufficient evidence to suggest that the event was most likely not due to natural causes. Determination of the actual cause of death made by coronial hearing.
Incidents of attempted suicide* at all New Zealand prisons:
2014/15: 2
2013/14: 25
2012/13: 7
2011/12: 6
2010/11: 23
2009/10: 31
* Intentional acts of harm to oneself which would most probably have led to death if there was no immediate intervention.
Source: Department of Corrections
Where to get help:
• Lifeline: 0800 543 354 (available 24/7)
• Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• Youth services: (06) 3555 906
• Youthline: 0800 376 633
• Kidsline: 0800 543 754 (available 24/7)
• Whatsup: 0800 942 8787 (1pm to 11pm)
• The Word
• Depression helpline: 0800 111 757 (available 24/7)
• Rainbow Youth: (09) 376 4155
• CASPER Suicide Prevention
If it is an emergency and you feel like you or someone else is at risk, call 111.
