John Roper responds to questions at his home in 2017 after he was placed on special leave.

The boss of one of New Zealand's largest prisons has been sacked after a major investigation revealed staff illegally spied on inmates.

Christchurch Men's Prison director John Roper, who was on a final warning, has been on gardening leave for 16 months.

He and two other senior staff – security manager John Cooper and residential unit manager Doug Smith – were placed on "special leave" when the Department of Corrections launched an inquiry into security issues at the jail in May last year.

The investigation – dubbed a "security review" – unearthed "extremely serious allegations" relating to a "number of substandard management and security practices" at the jail, according to a summary of the findings.

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On Thursday, Corrections confirmed Roper's employment at the jail had ended. Stuff understands he was sacked. Roper declined to comment on Thursday.

DAVID ALEXANDER/STUFF Christchurch Men's Prison boss John Roper has been fired after being on "special leave" since May last year.

Two other staff, understood to be Cooper and Smith, remained "subject to employment investigations", regional commissioner Ben Clark said.

"We demand a high standard of conduct and integrity from all our employees and, if any staff don't meet the standards, we take appropriate action."

Stuff was leaked two pages of the department's draft findings last year, which revealed elite guards from the prison's Site Emergency Response Team (SERT) illegally spied on inmates using unauthorised covert devices.

The chief ombudsman, following a complaint from Stuff, later ruled a summary of the complete findings should be released, which Corrections made available in March.

The summary says the department's investigation report, which was passed to police late last year because of the "serious nature of the findings", "indicates non-compliance with legislation and security procedures", including the Corrections Act.

"The issues identified as part of the security review relate to failing to action a number of intelligence reports and telephone monitoring disclosures, use of cellphones and the use of covert audio devices," the summary says.

The review found staff gave "favourable treatment" to some inmates who, according to the leaked documents, were given cellphones during covert operations. Cellphones are contraband in prison.

Police are investigating a "narrow" part of the review's findings, understood to relate to the use of the listening devices.

Prison sources say Corrections officials are concerned information gathered at prison using eight covert listening devices – bought between October 2014 and August 2016 – might have led to or contributed to prosecutions or inmates being denied parole.

In April, senior prison guard Alastair Wood was placed on "special leave" after he allegedly tried to interfere in the investigation by contacting one of the witnesses. Wood has since returned to work, but an employment investigation is ongoing.

In April last year, Stuff reported Roper and Smith received warnings after an inmate at the prison self-harmed while left on his own for several hours in an exercise yard in 2016.

The incident took nine days to be flagged at a national level after local staff failed to report it.

Roper previously warned staff they would "face the consequences" for leaking information to the media. He still had use of his work car during the 16 months he was on leave. Stuff understand's Roper's annual salary was more than $100,000.

On Thursday, Clarke said acting Christchurch Men's Prison director Dave Pattinson would return to his position as the prison director at Northland Region Corrections Facility. Jo Harrex would take over as acting prison director.

Christchurch Men's Prison houses about 900 inmates and is one of the country's largest jails.