An officer has suffered serious head injuries in an assault at Rimutaka Prison.
The Corrections Association says the officer, who has 17 years experience, stepped in when a prisoner was being assaulted by a group of inmates this morning.
The Department of Corrections said the low security prisoner and staff member are being treated in hospital for their injuries.
Corrections chief executive Ray Smith said the the staff member went to the aid of the prisoner who was being assaulted by a group of other prisoners.
"Other staff who intervened to assist did an excellent job to bring the situation quickly under control," Smith said.
"Those staff have been treated for injuries by prison Health Services."
He said at no time was the safety of the public at risk and the incident was limited to one secure unit.
"The incident has been referred to the police for investigation, and criminal charges are expected to be laid."
The union says the attack is another example of why officers should be carrying the proper safety tools such as pepper spray and Tasers.
"It has been just over two years since J Palmer was killed at work from serious head injuries sustained in a prisoner attack and yet we are still walking the landing with nothing but our wits and a cotton uniform," president Beven Hanlon said.
"It is time our safety was taken seriously. It is one thing to keep investigating these incidents and charging prisoners for attacking staff but that is little comfort to the family and friends of these injured staff," Hanlon said.