A prisoner is being treated at North Shore Hospital following a suspected assault at Auckland Prison this afternoon.
Police are investigating the incident, which has reportedly left the male prisoner seriously injured.
In a statement, Northern Regional Commissioner Jeanette Burns said violence is "always a risk" as Corrections manages "some of New Zealand's most difficult and challenging citizens".
"Many offenders resort to violent behaviour as a means of resolving issues and of expressing themselves," she said.
"Prisoners can be volatile and unpredictable and many have long histories of antisocial behaviour."
Ms Burns said when such violence occurs, "prisoners will be held to account for their actions".
The circumstances surrounding the alleged assault are not yet known.