Prisoners assaulted staff members at Otago Corrections Facility on Saturday, putting the prison into lockdown for several hours.
Acting prison director Lyndal Miles said three prisoners assaulted three staff at the facility in the afternoon.
The three staff members were assessed by health staff on site. One staff member required further medical treatment for "non-life-threatening injuries".
"Our thoughts are with our staff and their families," Ms Miles said.
Staff followed standard procedures by locking down the site and the situation was fully under control, she said.
Police had been advised and would investigate, she said.
Violence against staff or other prisoners was not tolerated in prison.
"Any prisoner who is seen to be fighting or using violent behaviour will be held to account ... Despite our best efforts and the extensive controls in place, unfortunately violence does happen in prisons."
