A Rimutaka Prison warden and prisoner are being treated in hospital for serious injuries after the prisoner was suddenly attacked by a group of other inmates this morning.
The warden had gone to help a low-security prisoner who was ''spontaneously'' assaulted by a group of other prisoners, Department of Corrections chief executive Ray Smith said.
Other staff intervened to bring the situation quickly under control. Some of them were also injured.
"The staff member and the prisoner's injuries are serious, and they are currently being assessed by medical staff," Smith said.
"I extend our thoughts and support to the families of the Corrections Officer and those injured in today's incident.
An investigation into the incident is underway.
"The incident has been referred to the police for investigation, and criminal charges are expected to be laid," Smith said.
He said the department would not tolerate prisoners using violence against staff or other prisoners, and the prisoners involved would be held accountable for their actions.
"Our staff carry out their work in the knowledge that assaults will happen. Prisoners are volatile and unpredictable and many have long histories of antisocial behaviour and resort to violence with little warning,'' Smith said.
"I extend our thoughts and support to the families of the corrections officer and those injured in today's incident.''
Frontline corrections officers receive specialised tactical and de-escalation training and personal protective equipment such as stab proof vests, batons and spit hoods are available for those working in high risk situations, he said.