Organised fighting was uncovered at Mt Eden Corrections Facility as early as mid-2014 - a year before it was made public - but a Corrections Department investigation into the violence never saw the light of day and no action was taken.
The department now says it was a mistake not to act on those early reports.
The oversight has annoyed Corrections Minister Judith Collins, who described it as "totally unacceptable". She is now seeking changes to the way ministers are notified about internal investigations.
Video footage of "fight clubs" within the Serco-run prison were posted online in July 2015, sparking an investigation by Corrections' chief inspector and eventually leading to the decision to end Serco's contract.
The results of Chief Inspector Andy Fitzharris' investigation were released yesterday, detailing widespread failings by the private prison provider including violence, drug use, haphazard staffing, and unhygienic conditions within the jail.
The chief inspector's report also sheds new light on an earlier investigation by Corrections "special monitors" in 2014 into organised violence at Mt Eden Corrections Facility (MECF).
A probation officer alleged "a significant amount of fighting" within the jail in May 2014, but when Corrections' Professional Standards Unit followed up the allegation, no evidence was found.
National Commissioner Jeremy Lightfoot ordered a further review, and sent in two special monitors to look at the prison's CCTV footage, interview prisoners, and check Serco's documentation.
The monitors' draft report in July 2014 said organised fighting was taking place between prisoners, that staff must have been aware of it, and that it occurred during periods of reduced staffing.
However, the report was never completed.
Its findings were not provided to Serco until May 2015, 10 months later. In addition, Corrections chief executive Ray Smith and then-Corrections Minister Sam Lotu-Iiga were not told about the report.
Fitzharris' inquiry appeared to point to a cover-up: "The [MECF] prison director has stated that he was told not to reveal the contents of the report to anyone."
Fitzharris went on to say that the draft report should have been finished, provided to Serco, and escalated to Corrections' senior management.
Asked to respond, a Corrections spokeswoman said the National Commissioner had been concerned about the evidential basis of the report. As a result, it was not finished or escalated and Serco were not notified.
That was a mistake, the department said.
"The department fully accepts the chief inspector's view, and that of ... Audit and Risk Committee members, that the report should have been finalised, formally provided to Serco and escalated to Corrections senior management, including the chief executive."
Collins also said action should have been taken "straight away".
"The draft report that I'm aware of was sitting with the National Commissioner for  months - a very untimely delay."
"And Serco management was actually asking for that report. So I don't know why that was not elevated to the chief executive or to the then-minister.
"What is clear is that it wasn't dealt with in a way that I would expect."
The minister now wants changes to ensure there is not a repeat.
She has appealed to the State Services Commissioner for greater ministerial powers, including access to draft investigations.
"I want to know what is going on. I think there is an opportunity for far more oversight."
HISTORY OF VIOLENCE
2009: An investigation uncovers fighting between prisoners at Mt Eden Corrections Facility, at the time a publicly-run prison. Changes are made to reduce the risk of further fighting.
2011: Serco takes over management of MECF
May 2014: A new investigation is launched into organised fighting at the prison.
July 2014: The investigation confirms organised fighting at the jail, and concludes that staff are aware of it. However, the report is never finished or forwarded to senior staff.
May 2015: Serco is finally told of the investigation's findings.
July 2015: Video footage emerges of fight clubs and drug use within MECF, sparking a major inquiry by Corrections' chief inspector.
Oct 2016: After an unsuccessful court challenge by Serco, the chief inspector's report is released, detailing frequent, vicious "fight clubs" within the jail.