The Department of Corrections says two prisoners have been hurt in an altercation at Rimutaka Prison near Wellington.
The department said the fight was contained by guards about 3pm on Tuesday and no staff or other inmates were injured.
Medical staff were called and the two men were taken to hospital for further treatment.
Police are investigating. Ambulance staff said one person had suffered a knife wound and is in a moderate condition.
Inmate coaxed down from roof
A prisoner who sat on the roof of Ngawha prison in Northland for most of Tuesday afternoon has come down following negotiations with staff.
The jail near Kaikohe went into lockdown after the man climbed onto the roof and refused to budge.
Prison manager David Pattinson said there was no chance of the inmate escaping, and no danger to the community during the incident.
Staff are trained to manage such situations and did an excellent job, he said.