Three Otago prison guards are recovering after being assaulted by an inmate.
The Otago Corrections Facility near Milton is one of the country's newest jails, and holds about 400 male prisoners from low to medium-high security.
Prison manager Jack Harrison said a prisoner assaulted three guards on Wednesday morning.
One was taken taken to hospital but has since been discharged; the other two were checked by a doctor.
The Department of Corrections would not explain the circumstances of the attack, but says serious assaults against staff are rare and extremely concerning.