A man has walked into the Whanganui police station and confessed to multiple sex offences against young children.

Gavin John Miles was unsure how many children he'd abused between 1990 and 2002, but told police he "hoped it was fewer than 50", a court has heard.

Justice Robert Dobson sentenced Miles to preventive detention in the High Court at Whanganui yesterday for three counts of sexual violation, and nine counts of indecency against children under 12, which he pleaded guilty to. The charges relate to two girls and one boy, the only three victims police were able to identify.

The victims could not remember the offending.

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Miles moved to Whanganui from Auckland, where the offending happened, because he was aware news of his offending on one child might have spread to an adult, defence lawyer Stephen Ross said.

No complaint had been made to police.

Miles then went into the Whanganui police station and gave a DVD interview, admitting all his offending.

He also agreed to a further interview with police at a later date.

Justice Dobson said the offending happened in West Auckland, by Henderson Creek, which Miles told police was his "hunting ground".

The victims, who were children playing in the area, were aged 4 to 8 years old.

The abuse involved groping, digital violation and "oral contact". Justice Dobson also spoke of Miles stripping naked in front of children, and showing them pornography.

"The summary of facts is drawn almost entirely from your admissions," he said.

The court also heard Miles was sentenced to five years prison in 2013 for offending against another 4-year-old girl on whom he had committed indecent acts.

He also had her commit indecent acts on him.

Mr Ross outlined mitigating factors, including Miles' full confession and early guilty plea.

He said Miles was "being sentenced for offending that he admitted to that would not otherwise have been discovered".

Miles was willing to take part in treatment and rehabilitation programmes, and had also sought help for "deviant thoughts" while in prison and was taking medication.

"He has expressed remorse. He clearly knows without ever really thinking it through that sexual abuse does ruin people's lives," Mr Ross said.

Justice Dobson considered both a finite prison sentence and preventive detention, but did not feel a finite sentence would be enough to protect the community.

He described the offending as "chronic" and said Miles did not understand the "sources" of his offending, and would be a danger to society unless "physically or chemically constrained".

He sentenced Miles to preventive detention with a minimum of five years in prison.