The Corrections Minister says he will be holding the private operator of Auckland's Mt Eden Prison to account after videos of inmate fights were shared online.
Several videos depicting violent scuffles between inmates, some with Black Power affiliations, have been filmed, using smuggled mobile phones, and uploaded on social media.
Warning: the video below contains graphic images, which may be disturbing to some viewers
Department of Corrections Northern Region Commissioner Jeanette Burns said 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.
Labour Party justice spokesperson Jacinda Ardern said the incident highlighted why the company should not be put in charge of New Zealand prisons.
"Serco are understaffing Mt Eden, making it dangerous for both staff, and prisoners."
"We've been raising this point for well over a year and with no response from the Minister."
She said only weeks ago, Corrections Minister Peseta Sam Lotu-Iiga praised Serco in front of a select committee for running what he said was the country's highest-performing prison.
Mr Lotu-Iiga said he still stands by the that.
He said while he was holding Serco to account over this incident, he would wait for an investigation before reviewing its contract.
"I'm looking forward to a review by the chief inspectorate, and until we get all the facts behind these incidents, we can't form any conclusions."
"But we will be holding them to account over their contracts, and in terms of the safety of prisoners and staff."
Last year's figures showed Mt Eden had more prisoner assaults in three years than any other Corrections facility.
But Mr Lotu-liga said inmate fights were not unique to the private-run prison, and happened at public prisons as well.
Serco Director of Operations Scott McNairn said in a statement that he did not underestimate the challenges of running the prison, but that for many inmates, violence was the norm.
"We do not underestimate the challenge of operating New Zealand's busiest remand prison."
"We hold 976 of the country's most difficult and challenging individuals. We manage 30,000 prisoner movements and 15,000 domestic visits each year."
He said security was tightened following the incident, including limiting access to exercise yards, and instigating more intensive searches of prisoners and cells.
"Everyone entering the prison is searched, regardless of their position or role."
Mr McNairn said the smuggling of cellphones into the prison was a real issue, with 27 mobile phones and 22 SIM cards confiscated this year alone.