The Sensible Sentencing Trust is sickened, angry and frustrated over the sentencing of a man responsible for one of New Zealand's worst cases of sexual abuse.

64-year-old Christchurch school caretaker and teacher Robert Selwyn Burrett has pleaded to 21 charges of sexual abuse of 12 primary school girls aged 5 to 12.

Victims Advocate Jayne Walker is calling for a Preventive Detention sentence to be imposed, in order to reflect the horror that he inflicted on his victims.

She said Burrett has committed a massive breach of trust as a caretaker and teacher.

"It's absolutely horrific to think that you can send your children off to school and find out that they've been sexually abused, raped, sodomized for two years. Absolutely atrocious."

Jayne Walker hopes the judge in the case will have the courage to make an example of him.

"We hope in this particular case that Preventive detention is given to this offender because he should never, ever be released again. Chances are, he has offended before - he seems to be quite a professional really.."