An inmate at Spring Hills Correctional Facility is in a stable condition at Waikato Hospital after an assault.
The prison's manager, Chris Lightbown, says prison officers responded immediately and called medical staff.
No one else was injured in the altercation on Thursday night. Police are investigating the assault.
Mr Lightbown said assaults like this are unacceptable, but inevitably take place from time to time.
Last year, the prison was the scene of a 10-hour riot, which caused about $10 million in damage.