Police have charged a man over an alleged assault that hospitalised an inmate at Paremoremo prison.
A 35-year-old has been charged with wounding with intent to cause grievous bodily harm and is scheduled to appear in the North Shore District Court in June.
Prison Director Andy Langley said a prisoner was taken to hospital on May 7.
Langley said staff had responded immediately and sought medical assistance for both the victim and perpetrator.
"The area in which the assault took place was secured as a crime scene and all evidence has been provided to police, including CCTV and On-Body camera footage.
"Corrections is also undertaking a review of the incident."
The prisoner was released from hospital back to Paremoremo prison on May 9, however he was returned to hospital on May 13 due to staff's concerns for his health, Langley said.
"We have a zero tolerance for violence policy, and any violence or assaults against other prisoners or staff is not tolerated in prisons," Langley said.
"Any prisoner using such behaviour will be held to account for their actions, including facing criminal charges."
Langley said they were constantly working to ensure prisons provided the safest environment possible for staff and prisoners.
"Our staff manage some of New Zealand's most dangerous people in a complex and challenging environment.
"Over 75 per cent of the prison population have convictions for violence in their offending histories, and more than 90 per cent have had a lifetime diagnosis of a mental health or substance abuse disorder."
Attempts had been made to contact the victim's next of kin, Langley.
"As the incident is subject to police investigation and for privacy reasons, no further information will be provided."