A prisoner in Otago Corrections Facility has been taken to hospital after an incident.
Police say they're investigating what happened at the prison near Milton on Friday morning.
Prison director Lyndal Miles said the prison is also investigating.
"No assault or violent behaviour is acceptable. However, it is an unfortunate reality that from time to time this will occur.
"Any prisoner who resorts to violence will be held to account."
In February, three prisoners assaulted three staff members at the facility.
Otago Corrections Facility opened in 2007 and accommodates up to 485 male prisoners, ranging from low to high-medium security.
- NZ Newswire