A babysitter who raped a girl and sexually violated her sister while on a supervision order has narrowly avoided preventive detention, despite being considered at a high risk of reoffending.

The Crown requested preventive detention, which can only be ordered by a High Court judge, for Matthew James Shirley, 26, in the High Court at Napier yesterday.

That would mean he would be sentenced to a minimum term, and only be released thereafter if authorities were satisfied he no longer represented a significant risk to the public.

Shirley pleaded guilty to charges of unlawful sexual connection, rape and two charges of indecent assault for the offending against two sisters, aged 10 and 11, in April this year.

He was sentenced to nine years and two months for the rape, with a minimum term of imprisonment of six years. Terms of three years for unlawful sexual connection and 18 months for each indecent assault were concurrent.

The offending occurred in Whakatane and he had previously been jailed for nine months for a similar offence in 2007.

In sentencing, the court was told how Shirley had been a friend of the girls' mother. She had decided to arrange a babysitter one night and he volunteered.

That day he drank 15 pre-mixed bourbon and colas, and not long after the mother left, violated the girls.

Shirley admitted the charges in June.

A victim-impact statement described how the damage that Shirley had caused "can't be measured". The girls had behavioural issues and didn't want their mother ever to leave the home, Justice Peter Woodhouse said.

Psychologist reports showed alcohol had been a "driving force" for Shirley's offending.

While acknowledging there was a risk, Justice Woodhouse deemed a lengthy minimum term of imprisonment as well as access to treatment would be appropriate punishment.

"You have never received controlled treatment for the alcohol abuse," Justice Woodhouse said.

"You have never received treatment to address the sexual offending ... You recognise the harm that you have caused. I regard that as particularly important."

Parole would also have the option of extended supervision on release and it was his first "three-strike" offence.

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