An Oamaru sex offender is considered so dangerous by Corrections they want to monitor him for the next decade - the maximum allowed by law.

Regan James Maindonald (27) was due to be released from Otago Corrections Facility in January following a 28-month jail term for having sex with a 15-year-old, but because of his high-risk status, authorities were keen to keep tabs on him.

An interim extended supervision order (ESO) was granted by the court and Maindonald was transferred to Christchurch.

The matter came before the High Court at Dunedin this week where Corrections argued the order should be put in place for 10 years.

The application also sought a year-long period of intensive monitoring - again, the maximum available period - through which Maindonald would wear an electronic tracking device and be accompanied 24 hours a day by live-in staff.

Justice Gerald Nation said he could only grant the ESO if he deemed the man exhibited a "pervasive pattern of serious sexual offending" and was at high risk of committing a future relevant serious sexual offence.

Maindonald made headlines in 2013 when he was sentenced to nearly seven months' imprisonment for groping three women - all in their 60s - at Moana Pool.

One of the women was in the pool supervising a group of intellectually disabled people when the man swam past her under water, brushing his hand against her backside, then returned to squeeze her right buttock.

He touched another victim on the lower legs and grabbed the third woman's right buttock and "squeezed it hard".

While Maindonald admitted what he had done, he told police "the ladies didn't seem to mind".

The indecent assaults saw him given a first-strike warning under the controversial three-strikes legislation; and two years later he received his second for having sex with a teenager.

At his sentencing in November 2015, the Dunedin District Court heard Maindonald had been warned not to take advantage of the girl but disregarded the advice.

He later brushed it off as a "set-up".

Crown prosecutor Marie Grills, acting on behalf of Corrections, told the court this week Maindonald would likely reoffend ''rapidly'' if released without supervision.

She said he had a "quite unusual degree" of sexual preoccupation and a lack of ability to control his impulses which was exacerbated by drug use.

Aside from his convictions, Grills said Maindonald had shown further concerning behaviour when he made sexual approaches to two vulnerable patients while at Wakari Hospital.

On another occasion, the court heard, the man called his case worker and threatened to rape her.

Defence counsel Sarah Saunderson-Warner accepted there was a risk of her client reoffending if he was to be released.

However, she questioned whether it was likely it would be a "serious" sexual offence and whether Maindonald's previous convictions actually constituted a pervasive pattern of behaviour.

She called clinical psychologist Craig Prince to give evidence.

He told the court Maindonald required both generic mental health treatment and offence-specific therapy.

Under an ESO, Prince said, there was less urgency for Corrections to give people the counselling they needed.

Justice Nation noted Maindonald would be imprisoned for seven years without parole if he committed another indecent assault - his third strike.

"It could be just touching somebody on the bottom or stroking a leg," he said.

"That should stop you doing that sort of thing in the future, shouldn't it?"

"Yes" Maindonald replied.

The judge said the crux of the matter was whether the man was at risk of "serious" sexual offending in future.

If the ESO was granted, Corrections planned to move Maindonald back to Dunedin where he would live at a special residence.

However, no such house had yet been found.

Justice Nation reserved his decision.