An inmate and a prison officer have serious injuries after being attacked at Rimutaka Prison.
The Corrections Department says the staff member went to the aid of a low-security prisoner who was assaulted by a group of other prisoners on Friday morning.
The injured men were being treated in hospital for serious injuries, Corrections chief executive Ray Smith said.
Corrections didn't reveal the nature of the injuries.
Other prison staff also required treatment for injuries after intervening to bring the situation under control.
Mr Smith says a prison investigation has been launched into the incident, and police have been informed.
Criminal charges were expected to be laid against the attackers, he said.
"Corrections take the issue of violence in prisons extremely seriously and our priority is staff safety. The Department will not tolerate prisoners using violence against staff or other prisoners, and the prisoners involved will be held to account for their actions," Mr Smith said.
"Our staff carry out their work in the knowledge that assaults will happen. Prisoners are volatile and unpredictable and many have long histories of anti-social behaviour and resort to violence with little warning."
He said the incident was limited to one unit of the prison, which remained secure, and there was no risk to public safety.