An investigation into unauthorised bugging at Christchurch Men's Prison is being referred to police.
A report by Corrections is understood to have found that prison staff used covert listening devices to intercept private communications.
Corrections national commissioner Rachel Leota told the Herald the report made "extremely serious allegations that will result in employment action" as well as the referral to police.
She said prison director John Roper and two other senior staff from the prison had been on special leave since the review got under way and had not returned to work.
"A review into compliance with specific security procedures began at Christchurch Men's Prison in May, led by the department's director of intelligence and operations, and police were made aware of this work," she said.
"There was no suggestion that the prison's security had been compromised in any way."
She thanked the staff who came forward to contribute to what was "an extremely thorough review".
"As part of this process some staff were recently given excerpts from the review which related to them, so they could provide comment," she said.
"The report will also be referred to police.
"We demand a high standard of conduct and integrity from all employees, and if any staff don't meet the standards required of them then we take appropriate action."