The Corrections Department was told about organised fights at Mt Eden Prison up to 18 months ago, the association that represents prison staff says.
Videos depicting organised bouts between inmates have been filmed using smuggled smartphones and uploaded on social media.
Warning: the video below contains graphic images, which may be disturbing to some viewers
Corrections Association spokesperson Beven Hanlon said, before the prison was privatised, the association made submissions saying such fights were the types of things that happened in private prisons due to lack of staff.
"In particular at Mt Eden they are really short-staffed. They are completely stretched, they'd have two or three staff if they're lucky doing the jobs of what in public sector would have five or six doing. Which means they're not supervising the prisoners."
He said 12 months ago Corrections conducted an inquiry after the Association alerted it to its suspicions fight clubs were happening.
"Haven't been told what happened with that inquiry; never saw the results of that inquiry."
He said an inmate who refused to take part in an organised fight in Mt Eden prison was thrown off a balcony and had his legs broken.
Mr Hanlon said it is so dangerous in Mt Eden, it can not be too long before somebody dies.
Department of Corrections Northern Region Commissioner Jeanette Burns said earlier that an investigation was underway to find out how the incident took place, and whether the safety of prisoners was compromised.
Ms Burns said cellphones were considered contraband, and she was working alongside Serco, the British-based global outsourcing firm that runs the prison, to see if those involved would face charges.
The prison's operations were contracted out by the Government and it has been run by Serco since May 2011.