Prison bosses knew of a fight club at Auckland's Mt Eden Prison for up to 18 months but did "nothing about it", the prison officers union says.
The allegations come after video footage emerged last week appearing to show prisoners at Mt Eden Corrections Facility (MECF) fighting. The footage was recorded on smuggled smartphones.
Yesterday Corrections Minister Peseta Sam Lotu-Iiga announced a "robust and thorough" review of the incidents.
Prison operator Serco has said an internal investigation is being carried out, and police have also been advised and may conduct their own investigation.
However, Corrections Association president Beven Hanlon said he had raised concerns about the prison 18 months ago.
Two people were appointed to investigate, but the association "never heard any results", and only recently saw the report, he told Radio New Zealand this morning.
"I can tell you now that on July 9, 2014, the chief executive, the national commissioner and the northern regional commissioner all received that report," he told the broadcaster.
"And it doesn't make nice reading. Particularly 12 months later when we're watching it on video."
The report "tells you everything you've seen on the videos that have been leaked onto YouTube", he said.
"We've got Corrections knowing that this stuff is happening and doing, clearly, nothing about it."
Serco "must know about it as well", he said, as people had gone into the prison to investigate.
"So you can't tell me Serco don't know Corrections did this investigation, and worked with Corrections on this investigation," he said.
Speaking on NewstalkZB this morning Mr Lotu-Iiga said he was "disappointed and really appalled" by what he had seen on the videos.
He had been unaware of the so-called fight club until Thursday when the videos emerged he said, describing it as coming "out of the blue".
Serco was aware of his disappointment he said, and "know they've got to investigate this thoroughly and get to the bottom of it".
Asked if Serco had breached its contract, Mr Lotu-Iiga said: "We're not sure yet, that's why we need to get an investigation going."On the face of it, it doesn't look good, but we need to find out and answer a few questions and take it from there."
He added: "On the face of it, these gentlemen are really breaking the rules, that's pretty clear, so ... we need to find out how they got hold of mobile phones, what circumstances are around it and make some decisions from there, but until we have a thorough investigation we can't form any views about it."
Serco had already taken "some steps to remedy" the incidents, he said, but the prison operator "needs to work harder".