Thirteen-year-old Jordan Nelson shot and killed his caregiver because she had stopped him from visiting his mother in Hawke's Bay, a court was told.

But he got it wrong.

It was Child, Youth and Family which did not allow him to see his mother, Justice Paul Heath said when sentencing Nelson in the High Court at New Plymouth yesterday.

Nelson is now New Zealand's youngest convicted murderer.

He pleaded guilty to using a .22-calibre rifle to shoot 50-year-old Rose Kurth, his de facto grandfather's partner, at their rural home at Okoki, near Urenui, on the afternoon of April 15.

Nelson was sentenced to 18 years' jail after Justice Heath decided the Waitara High School student, suspended from school for smoking marijuana, was too young to be given the mandatory life sentence.

The finite 18-year sentence allows Nelson to apply for parole in six years, when he will be 19.

Nelson admitted to police that he shot Kurth in the back of the head with the rifle at close range while she sat at the kitchen table completing a jigsaw puzzle.

He had no answer to why he put the rifle in his sleepout earlier in the morning while his grandfather, Kerry Lock, was walking the dogs.

After dragging Kurth's body into a bedroom, and ransacking Lock's room, Nelson drove to Waitara in his victim's car.

The court heard how Nelson did not have a relationship with his father because his mother had a restraining order against him.

Kurth's family is now calling for the Crown to appeal against the sentence.

"We felt the focus of today's decision hinged largely on the defendant's young age and worry that it could be setting a dangerous precedent for future cases," a family statement said. "We feel that unfortunately justice wasn't done and that Rose's death was for nothing."

Detective Senior Sergeant Grant Coward said there were no winners in the tragic case but with Nelson pleading guilty, the family had been spared a trial.

Earlier, Justice Heath said Nelson's psychiatric assessment showed he had no mental health problems but had been exposed to domestic violence - making it clear this was not while with Mr Lock and Ms Kurth.

Lock is the former de facto partner of Jordan's grandmother and had known him since he was a baby.

He took care of the boy, who was the subject of a care and protection order, through a written agreement between his family and CYF.

Nelson will be sent to a secure youth residence until his 16th birthday, and is then likely to be transferred to an adult facility.

MEMORIES HAUNT VICTIM'S PARTNER

The man Nelson called Granddad still suffers flashbacks after finding his partner's bleeding body in the rural Okoki home the three shared.

"I thought my world had ended," Lock said as he read his victim impact statement to the High Court in New Plymouth yesterday.

Jordan Nelson was sentenced to 18 years' jail for the murder of Rose Kurth.

Lock told the court he returned from shifting the stock near their Piko Rd home to find two bloody drag marks leading into the spare room.

He opened the door to find Kurth lying on the floor with blood coming out the back of her head, her mouth and ears.

The ongoing effect of her horrific death at the hands of the boy he thought of as a son was immeasurable, he said.

"I suffer from vivid flashbacks that randomly affect my ability to maintain day-to-day tasks."

His voice broke as he said: "I miss Rose like crazy and all I can do is kiss the photo beside the bed every day."

Two of Kurth's daughters also shared the effects her death had had on them.

"What Jordan has done has torn my family apart," Jimmy Kurth said.

Her children would now never have a relationship with their grandmother.

She spoke of recurring nightmares and her fear of being alone.

"I don't understand how Jordan could be so selfish and nasty, all because he was grounded and it was for a serious matter.

"I hate what you have done, you stole from me. Nothing can fix the pain you have caused," Kurth said.

Their statements were two of three read to the court by their writers yesterday.

Several others were also before the court.

- © Fairfax NZ News