CHRISTCHURCH - David John Gay - described as "evil filth' by the mother of the little girl he abducted and sexually assaulted - knows he may serve the rest of his life in prison.

The hearing in the High Court at Christchurch indicated that his open-ended preventive detention sentence was a foregone conclusion, but Justice John Fogarty heard submissions on the non-parole period before imposing eight years.

Defence counsel Michael Starling said that Gay, now 44, had met people serving preventive detention sentences during his years in prison.

"There are people in Paparua who have served 20 years. He knows that no matter what period you set, he can effectively serve the rest of his life in prison," Mr Starling told Justice Fogarty.

"Often people on this sentence don't get out. The Parole Board is incredibly tough."

Gay has a history of sex offending against young girls. One of his earlier victims was aged two years.

In mid-2007 he had not long been out of jail for sexual offending in Invercargill. He was living in the Christchurch suburb of Linwood and an extended supervision order had recently been imposed.

But some nights he would go out prowling and entering houses. One night he found a woman asleep on a sofa and sat watching her until she awoke.

He entered another house about 3am on July 1 through a back door which had been left unlocked so a couple using a sleep-out could have access to the toilet during the night.

There were 12 people sleeping at the address because the family had gathered for the grandmother's 70th birthday.

Gay took money from a handbag and then took a sleeping seven-year-old girl out of her sleeping bag and carried her out the door onto the decking where he removed her underwear and indecently assaulted her.

When she woke and screamed he threw or pushed her onto the ground before running off.

In court, he pleaded guilty to charges of abduction, indecent assault, breach of parole, two burglaries, and unlawfully interfering with a car.

The girl's mother read the family's victim impact statement in court. She said it had broken her heart not to be allowed to be with her daughter during the investigation and the upsetting rape examination.

"The terror, shock, fear and heartbreak that I felt as a mother when I first held her after the attack will never leave me.

"The disgust and hate we feel for this person is so strong it, too, will never leave us. We have all been touched by the evil filth of this offender."

The family lived outside the city and had come in for the family gathering. "Christchurch has become a place full of evil to us, and will never be the same."

The family had increased security and now kept doors locked even during the day.

Mr Starling urged Justice Fogarty to impose a five-year non-parole term, but crown prosecutor Catherine Dalziel sought a term of at least 10 years.

Justice Fogarty said: "It seems to me this man simply doesn't have the normal mechanisms of self control and moral sensibility that normal people in the community have."

He described Gay's offending as opportunistic and impulsive. "The fact that you have sexually offended against a child as young as two years, suggests to me that you are likely to be suffering from a mental disorder as defined under the Mental Health Act."

He was satisfied that there was a pattern of serious offending, and it was escalating. "Obviously, this form of sexual offending is of great seriousness and harm to the community."

He said he was disturbed to hear that authorities only arranged treatment for such prisoners when their non-parole term had expired. He hoped that Gay would be given some opportunity to attend programmes before the term expired, but he also noted that the man had already attempted the Kia Marama programme for sex offenders.

Outside the court, the girl's family welcomed the sentence. It was excellent to know Gay was in prison and could not offend against other little girls, said the mother.

- NZPA