UPDATED: 6.34pm A man is critically ill in Auckland Hospital after what's understood to have been a stabbing at Auckland's Paremoremo Prison this afternoon.

Another prisoner required medical attention but did not have to leave the prison. 

Corrections spokeswoman Jeanette Burns has confirmed an incident involving two prisoners occurred, and said Police are now investigating.

She said the prison houses some of New Zealand's most difficult and challenging offenders, and violence is always a risk.

Ms Burns said many prisoners resort to violent behaviour as a means of resolving issues and of expressing themselves.

Police were called to the prison at 2.44 this afternoon.

Auckland Prison, also known as Paremoremo, contains New Zealand's only specialist maximum-security prison unit.