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The Department of Corrections says it is not hiding the real number of assaults by prisoners at the Otago Corrections Facility at Milburn - despite the union recording 128 assaults since its 2007 opening and the Corrections department noting only seven.
Beven Hanlon, president of the Corrections Association, the union representing corrections officers, said the Corrections Department did not want the public to know what it was really like to work in a prison.
Mr Hanlon, who is an officer in the North Island, said the situation was one of "holding on for grim life" for officers, and it would only get worse with double bunking, Double bunking - the practice of two prisoners sharing one cell - will begin at the Milburn prison by February with the prison's official capacity expected to rise from 335 to 485.
The prison now employs about 200 staff.
The union has been holding stopwork meetings with its members about double bunking and Mr Hanlon said 99% of officers were unhappy with the situation.
The Department of Corrections is still involved in negotiations with the two unions - the Corrections Association of New Zealand and the Public Service Association - about increasing the Milburn prison's maximum capacity.
"[The department] likes to call it double bunking - We call it overcrowding," Mr Hanlon said.
The incidents the union had recorded, from the opening of the prison to the end of July, included an assault by a prisoner where another prisoner was punched in the head and his clothing set on fire.
He did not receive serious injuries.
In another assault, a female officer was punched three times in the head.
In March, two Black Power members attacked a Mongrel Mob member by throwing hot water mixed with jam and sugar over the victim's head and upper body and then stabbing him with hand-made weapons in the chest and back.
Also in March, a prisoner was stabbed in the cheek with a sharpened pencil.
"The safety of our staff is one of our highest priorities and we reject any suggestion of a cover-up on the number of staff assaults," Otago prison manager Jack Harrison said in an emailed response.
He said that in the past, the union included the category of "other assaults" when discussing the number of assaults in prison.
Included in this category were minor altercations, such as a prisoner spitting on an officer.
This category was not collated by Corrections.
Mr Hanlon rejected this, saying there had to be some sort of malice in the incidents which the union reported and many more minor incidents were not reported.
Corrections southern region assistant regional manager Ants Howie said, in answer to a Official Information Act request, the department took the issue of violence in prisons "extremely seriously".
"Prisons can be volatile environments and violence is a risk.
While one assault is one too many, some prisoners have long histories of antisocial behaviour and resort to violence with little warning."
Corrections records assaults on the department's Integrated Offender Management System.
All notifiable incidents were subject to some level of investigation.
They were often, but not always, reported to the police.
Mr Harrison said Corrections recently completed a staff safety project which resulted in extra protective equipment, such as stab-proof vests, being provided to staff working in high-risk situations.