Staff had been attempting to return prisoners to their cells on Boxing Day when an officer was hit in the face, Otago Corrections Facility prison director David Miller said.
Other staff at the prison near Milton in Otago responded immediately to bring the situation under control, he said.
Mr Miller said the officer was seen by on-site medical staff and taken to hospital for further medical attention.
He was recuperating at home and was being supported by staff and a welfare team.
Mr Miller said police had been notified after what he described as a spontaneous assault in the high-security unit.
The prisoner was placed on directed segregation and had his security classification reviewed, Mr Miller said. He would be transferred to the maximum-security wing at Auckland Prison.
"Corrections officers come to work every day to keep New Zealanders safe and should expect to go home safely at the end of every day," Mr Miller said.
"Assaults on my staff are totally unacceptable and will not be tolerated in any circumstances, and perpetrators will be held to account.
"Many of the prisoners our staff work with can behave unpredictably and act without warning."
Mr Miller said more than 80% of the prison population had convictions for violence in their offending histories.
"The reality is the threat of violence is something we cannot eliminate entirely, but we are constantly working to ensure our prisons provide the safest environment possible for staff and prisoners."
Corrections had trained staff in de-escalation techniques and communication skills, and provided stab-resistant vests, pepper spray and body-worn cameras.
