Private prison operator Serco has gone to the High Court challenging a draft report into allegations of organised fighting and other issues at Mt Eden Prison, it has emerged.

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The report by the Prison Inspectorate was finished last month and is being reviewed by the head of Corrections.

Serco is unhappy with it; a High Court document obtained by RNZ News says it does not think it has had enough opportunity to comment and respond.

Reports into claims of violent attacks on prisoners and video footage that was able to be posted online of prison fight clubs have been ongoing for months.

Control of Mt Eden Prison was taken off Serco in July after the allegations surfaced.

The report has not yet been made public, and the timeframe for its completion has had to already be extended by two months because of the high number of complaints from prisoners and their families.

The investigation was done in two parts.

The first looked at the brawl itself and whether a fight club was operating at the prison.

The second part was designed to see if controls that were supposed to stop prison violence were good enough, and look at how inmates were accessing cellphones.

Last month, the report was sent to Department of Corrections chief executive Ray Smith to be reviewed.

RNZ asked Serco why it had gone to the High Court and why it was challenging the process used to complete the draft report.

Serco was also asked why it felt it had not had opportunities to adequately comment or respond, as mentioned in the court minute.

In a statement, the company replied:

"Serco is committed to work with the Department of Corrections on the finalisation of the report and its ongoing contractual commitments at Mount Eden Corrections Facility.

"As the proceedings are still before the court, at this stage, we can't comment further."

The Department of Corrections has confirmed Serco filed a judicial review a few weeks ago, and says the review is being considered by Corrections' chief inspector.

High Court judge Justice Simon France said he would not open the court file for RNZ because it would compromise the ability of Serco to challenge the report and obtain redress.

Labour Party MP Kelvin Davis said today the fact Serco was challenging the report dispelled his concern that the investigation would be a whitewash.

Mr Davis said he thought the investigation would be a once-over, but that was not the case.

"I'm happy to admit I was wrong there, but I've also got to give them a brickbat because it shouldn't have taken them so long to have picked up that there were problems."

Mr Davis told Morning Report the prison inspectorate's investigation had been thorough enough to concern Serco.