Corrections wanted an Oamaru sex offender under its supervision for a decade — instead he will be free in a couple of months, a judge has ruled.

Justice Gerald Nation told the High Court at Dunedin last week that 27-year-old Regan James Maindonald was "obviously" likely to commit further sexual offences.

However, he said that would probably be indecent acts rather than anything more severe.

Accordingly, the judge declined Corrections' application for a 10-year Extended Supervision Order along with the one year of intensive monitoring that would have put Maindonald under their scrutiny 24 hours a day.

While the application had been pending, the man had been in a special residence within Christchurch Men's Prison but separate from the general population.

Now he will be free within months without oversight. The court's ruling has exasperated the mother of Maindonald's most recent victim and his own mother, who says he is being "set up to fail".

Maindonald recently completed a 28-month jail term for having sex with a 15-year-old girl and was incarcerated for nearly seven months in 2013 when he groped three women — all in their 60s — at Moana Pools. One was supervising a group of intellectually disabled people when the man swam past her under water and squeezed her buttock.

The mother of Maindonald's teenage victim was astounded a judge could find a high likelihood of future offending and yet make no order to keep tabs on him.

"The system is broken," she told the Otago Daily Times.

"I think the next opportunity he has, he's going to do a similar thing."

Her daughter, she said, had left Dunedin and "built a wall around herself" since the episode with Maindonald.

"If he reoffends she's going to live through it all again and feel so so bad she wasn't able to protect that next person," the woman said.

As well as her daughter's future she was also concerned for Maindonald and said she hoped he received the treatment he needed.

Lois Maindonald, his mother, shared that sentiment. Although she did not want him under the spotlight of Corrections for a decade, she said leaving him to his own devices could be catastrophic.

Her son needed medication for his psychological issues and things went downhill fast if he did not take them, she said.

Lois Maindonald had been in touch with mental-health services in Christchurch to try and spark them into action.

"I can just see it all going wrong," she said.

In making his ruling last week, Justice Nation reminded Maindonald he had committed two "strike offences" under the three-strikes legislation and another would see him serve the maximum jail term that that charge carried, without parole.

"You need to understand just how serious the situation is for you," he said.

Lois Maindonald hoped so too.

"All he's got to do is rub someone's leg the wrong way and he's looking at seven years," she said.

"And there's a good possibility of that happening."

Maindonald's decline, she said, came in his late teens when he started using legal highs. Combined with his mental illness, "they turned his brain to mush".

Lois Maindonald said she simply wanted a future for her son that did not involve him flitting between prison and mental health wards.

"He's a really nice kid when he's on his meds," she said.

There was little chance of Maindonald returning to Oamaru to be with his family because of his previous behaviour with them.

It had been about two years since Lois Maindonald had seen him.

"He's like the son I haven't got," she said.

"I need to just love him from a distance, because I've just gone through so much."