A prison inmate raped a woman while on release to work in what has been described as the worst incident in the programme's 50-year history.

Inquiries found Vernon Neil Bugden, 35, coerced a woman into meeting him for sex while he worked unsupervised for a business on the outskirts of Christchurch.

During one of the woman's visits, in May last year, an argument erupted and Bugden raped her in a car.

He had been refused early release from Rolleston Prison by the Parole Board a month earlier who noted his "troubling propensity to inflict violence upon women".

The incident sparked a major internal inquiry by the Department of Corrections, which identified several failings and saw some inmates recalled to prison.

"This is by far the most serious incident that has occurred involving prisoners on release to work," Corrections Services southern regional commissioner Ian Bourke said this week.

Bugden admitted several charges, including sexual violation by rape, when he appeared at Christchurch District Court on September 10. He was remanded in custody ahead of sentencing on October 30.

A judge granted access to court documents relating to the case this week.

They show Bugden coerced the victim, whose name is suppressed, into meeting him several times while he was employed outside the wire as part of the department's release to work programme.

At the time, he was nearing the end of a two-year, eight-month stretch in prison for injuring with intent to injure.

Bugden did general labour from Monday to Saturday for waste water treatment company Oasis Clearwater, which had helped train prisoners.

He was allowed to cycle to and from work and had the responsibility for unlocking the premises in the morning and locking up in the evening.

During the day he worked in an area with only one other employee, predominantly with no direct supervision.

Bugden's victim visited him at work on Saturday mornings and they would often have sex. She brought food and a cellphone so that he could contact her. Bugden stored them in his personal locker on site.

On Saturday, May 3 last year, the woman visited him about 10am. It was his birthday and she brought with her some food. The pair began arguing and the victim said she was going home and did not want to be with Bugden any more. When she went to her car he pushed her inside and raped her.

Bourke said Corrections launched a major review of the incident and release to work as soon as police revealed they were investigating a complaint from the victim.

Release to work allows minimum security prisoners nearing the end of their sentences to work in the community to help them gain employment when they are released.

Bourke said the inquiry found Bugden met the criteria for the programme, but more consideration should have been given to his community based risk, the fact that he continued to deny his offending and his lack of motivation to address the offending.

The Parole Board refused Bugden early release from prison a month before the incident because he remained an "undue risk" to the community.

The board's report notes that Bugden had a "troubling propensity to inflict violence upon women".

"There is a reference in some of the material before the Board that Mr Bugden's risk is high and he may not have addressed his violent tendencies."

Bourke said the inquiry found that Bugden went to "significant effort to hide the fact the victim was visiting him". He was not supervised at all times, as he should have been by the sponsor.

Oasis Clearwater no longer takes prisoners involved in release to work.

As a result of the inquiry, the rules around release to work were "significantly strengthened" and suitability guidelines were reviewed, leading to some prisoners being recalled to prison, Bourke said.

Other changes included:

- Sponsors must pick up inmates and drop them back off at the prison.

- The frequency of release to work site checks have increased.

- Inmates on release to work on Saturdays must return to prison quickly.

Further changes to temporary release, which includes release to work, have been made since December last year. All prisoners going outside the wire are required to wear GPS bracelets unless they are accompanied by Corrections staff.

In 2014/2015 there were 466 prisoners on release to work across New Zealand.