Unlike the first assault, this week's incident was not thought to be gang-related.
The Department of Corrections yesterday confirmed a prisoner was assaulted by two others on Wednesday.
The prisoner was stabbed with a pencil.
"This prisoner was taken to hospital where he received appropriate medical care and was returned later that day," OCF manager Jack Harrison said.
Prisoners were entitled to have writing materials such as pencils, he said.
Mr Harrison said staff acted immediately when they became aware of the altercation and separated the prisoners.
"Unfortunately, incidents such as this do occur in prisons occasionally.
"Prisoners can be volatile and quick to anger. When this occurs, our staff will step in."
Mr Harrison said he had no reason to believe the incident was gang-related.
Police had been notified and were investigating, he said.
The incident follows the stabbing of the Dunedin Mongrel Mob president by two members of rival gang Black Power at the prison on March 8.
