Otago Corrections Facility prison director Dave Miller said the incident happened yesterday morning.
A prisoner was removed from his unit and placed on a management plan after he assaulted another prisoner, Mr Miller said.
The assaulted prisoner was seen by the prison health team for grazes to his face and nose before being returned to his cell.
"We have a zero tolerance for violence policy, and any violence or assaults against other prisoners or staff is not tolerated in prisons. Any prisoner using such behaviour will be held to account for their actions, including potentially facing criminal charges," Mr Miller said.
A police spokeswoman said a report of an assault at Otago Correctional Facility was received about 3.45pm yesterday and officers were dispatched.
An ombudsman report conducted in 2022 and released in August this year found violence in the prison had decreased, along with its population.
However, both prisoners and staff were concerned about safety.
A survey of 170 inmates found 61% had been victimised in the prison and 67 respondents said they had been assaulted.
About a third of those assaulted had reported it, the report said.
