A prisoner was talking to a staff member at Paremoremo Prison and "out of the blue" started throwing punches, says the Corrections Association.
Two guards have been injured in an attack at Auckland's Paremoremo Prison.
Prison Director Andy Langley said a prisoner assaulted two staff members at Auckland Prison just after 9am on Tuesday morning.
One staff member was left with a fractured nose, minor facial injuries and damaged teeth, and another with scratches.
"The staff acted quickly to ensure that the prisoner was restrained, and prevented further harm.
"Both staff were treated by our Health Services staff on site."
He said the prisoner involved was placed on directed segregation and later relocated within the prison.
"Police have been informed, and we'll assist their investigation however we can.
"Our thoughts are with the staff affected and we will continue to offer them support as required."
He said violence is not tolerated in the prison under any circumstances.
Detective Sergeant Ross Clapp, of Waitemata Police, confirmed police are dealing with an assault complaint after a corrections officer was attacked by a prisoner at Auckland Prison.
"Police are taking the matter very seriously and are in the process of reviewing CCTV footage and speaking to a number of witnesses," he said.
Paul Dennehy, national vice-present of Corrections Association of New Zealand, said the incident took place in the low-security West Wing.
"A prisoner was moving about the wing and talking to staff by the guard staff base area before he all of a sudden started to throw punches.
"The staff member who has suffered a fractured nose fell to the floor and another colleague has got involved. With the help of other staff they have brought the offender under control.
"There was no build-up or warning from body language. It was completely out of the blue."
Dennehy said staff responded fantastically and there was nothing else that could have been done to avoid the attack.
He said attacks on prison staff are happening more frequently because of the increasing size of the prison population.
"This brings stress upon our members, the department staff and the prisoners themselves because they are all in places that weren't designed for the sheer volume of numbers that we have got in them."
He said new legislation that will allow prison staff to carry pepper spray is a major step forward for the department.
"It is a major step forward for our staff in terms of not physically having to intervene but to keep a bit of distance between themselves and the prisoner."
He said training for the use of pepper spray is currently being implemented at one site, and will later be rolled out to prisons across the country.
"The department is always looking at other jurisdictions across the globe to see what techniques, tools or technology they have available to them.
"Some of this stuff is fantastic and we should mirror it, and some of the processes and procedures we should never go near.
"The department is moving in the right way in terms of that and looking at other options because our only ability to control a violent prisoner at this stage is to use physical force.
"The outcome of that is prisoners are getting hurt, staff are getting hurt and if we can try and minimise that then it will be better for everyone."
Police and Hato Hone St John responded to at least two crashes in Auckland last night.