A Dunedin sex offender subject to ground-breaking restrictive measures has told a health assessor there are more child victims police do not know about.

Glen Anthony Douglas, 25, became the first person in the country to meet the criteria for a public protection order (PPO) when Justice Nicholas Davidson made the ruling just before Christmas.

The legislation was introduced for offenders who have a "very high risk of imminent serious sexual or violent offending" if released into the community.

It means Douglas will be moved to the specially built "Matawhaiti Residence" beside but separate from Christchurch Men's Prison, with its own secure perimeter fence.

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The $8.9million facility, which will eventually have the capacity to hold 24 people, will be his home for at least the next year.

That could change if police press charges over confessions Douglas made in an interview with health assessor Debbie McLean.

In a report before the court, she wrote that the prisoner had made no progress on the Kia Marama child sex offender programme while behind bars.

"His desire to engage in sexual activity with children has not diminished," Ms McLean said. And there was more.

"Mr Douglas has disclosed undetected sexual offending, non-penetrative, against three male children, two aged under 12 years and one aged 12-16 years."

The ODT questioned police over whether the admissions were being investigated but a spokeswoman said the officer in charge of the case was off until mid-January.

However, she did confirm police were able to act on statements made under such circumstances.

"Reports of child safety concerns can be received by police from any source, and we assess and review all information gained from inquiries to determine what action is appropriate," the spokeswoman said.

Douglas has been locked up on "interim detention" and the PPO stint is due to start tomorrow.

But, conceivably, if police decide to lay new charges, he could be back among the regular prison community as his case runs through the court.

Douglas was jailed for three years on two counts of sexual connection with a young person in 2013.

The charges came after he groomed a 13-year-old boy using multiple identities on Facebook and sexually abused him over two meetings before the victim's mother found out.

He was due to be freed on August 29, but Corrections applied for the PPO a couple of weeks beforehand.

His lawyer, Sarah Saunderson-Warner, argued at the hearing in the High Court at Christchurch in November that her client could be released on an extended supervision order with intensive monitoring.It would effectively mean Douglas lived in the community under 24-hour surveillance.

Ms Saunderson-Warner said he could undergo psychological and pharmaceutical treatment and would have the chance to show he was not a risk to the community.

If he struggled, Corrections could apply for a PPO at that point, she said.

Justice Davison was not convinced.

"The need for potential victim protection is paramount," he said.

The judge said any form of detention other than under a sentence was "inherently troubling" but believed he had no choice.

"It is reserved for those cases where there is really no option, to avoid the very high, serious and imminent risk posed by the individual. Mr Douglas does pose that risk," he said.

Under the PPO, Douglas' case will be reviewed every year and Justice Davidson stressed his ruling did not mean the 25-year-old would be in the facility for a lengthy period.

Matawhaiti Residence

• Within the grounds of Christchurch Men's Prison and has its own perimeter fence.

• Capacity for six people.

• Capacity for 24 once complete.

• Each unit is separate and contains a kitchenette, bathroom, laundry, living room and bedroom.

• A fertilised area is designated as a vegetable garden.

• Legislation says: "persons who are detained in a residence under a PPO should have as much autonomy and quality of life as possible, while ensuring the orderly functioning and safety within the residence".