Organised fighting between prisoners was happening at least once a week while Serco was running Mt Eden Corrections Facility, the chief inspector of prisons concluded.
Corrections will publish the inspector's full report shortly, after a judicial review sought by Serco was dismissed.
In a judgment published today, Justice Karen Clark of the Wellington High Court noted that the chief inspector believed organised fighting was happening at least once a week during certain periods.
Serco told the chief inspector that organised fighting usually took place in cells out of sight of prison officers and CCTV, or the exercise yard during unsupervised periods.
Other prisoners in the same unit sometimes deliberately distracted prison officers, and injuries from the fighting could be serious.
There were five confirmed and two probable instances of organised fighting in the less than four weeks between June 11 and July 4 last year.
Multiple prisoners interviewed as part of the investigation said the fights had happened as frequently as daily.
Serco had argued that such interviews could not inform the investigation.
"It is telling that in its response to the final draft report Serco accepted the finding that, 'it is likely that organised fighting in the form of contender fighting or fight club was occurring at least once a week during certain periods'," Justice Clark wrote.
"Yet now takes the position that the finding was not available to the chief inspector other than on the basis of objectionable oral information."
The review concluded that the chief inspector's investigation was thorough, consistently objective, and balanced.
"To my mind the report itself is measured and temperate and Serco's description of it is wholly unwarranted. The investigation was fair and the report is without error."
The Department of Corrections invoked a "stepping in" clause in Serco's contract in July last year and in December confirmed the British-based company's contract would not be renewed at a break point in March.
Serco then launched legal action, claiming the chief inspector's investigation had departed from the terms of reference, contained errors of law, and had omitted relevant factors.
Last night, Serco chief executive Mark Irwin said the company noted the High Court decision and accepted it.
"We have always accepted the recommendations of the report, but brought the proceeding because we had genuine concerns about the process.
"We have acknowledged that under our management, following a period of good performance, standards in Mt Eden slipped in early 2015, and we have apologised for this."
Serco was supporting Corrections' management of Mt Eden by providing staff and other transition resources, Irwin said.
It continues to manage the Wiri Prison in South Auckland and that had been operating well, he said.
"All prisons have to deal with challenges, and we are grateful for the partnership of the Department as we work together to address them."
Serco had been in charge of Mt Eden prison since 2011, having won a $300 million, 10-year contract.
In April, the Government announced it has charged Serco $8 million to cover the costs of resuming control of the prison.