Serco outperforms most public jails in comparison tables released in bid for greater transparency.
Private prison operator Serco is outperforming most public prisons in league tables which measure prison escapes, positive drug tests and rates of violence and rehabilitation.
The Corrections Department published the performance grades for New Zealand's 17 prisons for the first time in a bid for greater transparency and accountability.
The tables showed six prisons had exceeded performance targets, eight were operating "effectively", and three needed improvement.
Mt Eden Corrections Facility, which is managed by British-based company Serco, was in the top tier of prisons, as it was exceeding its targets on reducing assaults, positive drug tests and complaints.
The operator had a patchy start to its $300 million contract at Mt Eden, with a poor record of escapes and prisoner-on-prisoner assaults.
Corrections Minister Anne Tolley said: "They've made a huge improvement over the last six months. They had a rough introduction, but there's some things we could really learn from them."
Christchurch Men's Prison and Rimutaka Prison, which had similar security profiles and populations to Mt Eden, were in a lower performance ranking in the league tables.
Hawkes Bay, Northland Regional Corrections Facility and Otago Corrections Facility were ranked as "needing improvement" because of a higher-than-acceptable rate of assaults between prisoners and on staff.
In its last quarter, Northland had a serious assault on a staff member and three assaults between inmates.
Corrections national commissioner Brendan Anstis said: "We've said that's too many and they can do better. We've taken a zero-tolerance approach and these places have more assaults than theyshould have given their size and complexity."
Prison reformers welcomed the release of performance figures.
Rethinking Crime and Punishment spokesman Kim Workman said the previous focus on risk management had led to a drop in escapes and positive drug tests, but an increase in unnatural deaths and assaults.
He said the measuring of security, safety and rehabilitation redressed this imbalance.
Mr Workman felt the next step would be to measure the "moral performance" of prisons, by assessing trust between staff and inmates, the level of family contact given to prisoners, and other social indicators.
While the new league tables were designed to increase transparency, Corrections refused to release the raw data to media.
The Herald was asked to submit an Official Information Act request to obtain these figures.
Asked why the information was not publicly available, Ms Tolley said: "Not each prison is the same, it depends on the classification of prisoners ... so there's some quite conscious weighting of different elements of it and this is the best way to show it - in a way which is easy to understand."
Ms Tolley said the initiative was not about shaming prisons into performing better, but was designed to help prisons to improve their performance and share ideas.
To December 2012
No prisons reached this level
Invercargill Prison, Manawatu Prison, Mt Eden Correctional Facility, Tongariro/Rangipo Prison, Waikeria Prison, Wanganui Prison
Arohata Prison, Auckland Region Women's Corrections Facility, Auckland Prison, Christchurch Men's Prison, Christchurch Women's Prison, Rimutaka Prison, Rolleston Prison, Spring Hill Corrections Facility
Hawkes Bay Prison, Northland Region Corrections Facility, Otago Corrections Facility
(Measures include unnatural deaths, breakout escapes and escapes from an escort, assaults, justified complaints, and positive random drug tests).