An Auckland teenager researched serial killers before stabbing his stepmother 15 times.

Michael McRae, 17, was sentenced on Tuesday in the High Court in Auckland to six years and seven months imprisonment after pleading guilty to attempted murder.

Photo: justice.govt.nz

Justice Matthew Palmer said McRae, who was 16 and six months at the time, was living at home with his father, stepmother and half siblings in Papakura.

On 5 August last year, his stepmother - Thai national Supanee Saensree, 31 - fell asleep on a chair, with her 14-month-old daughter in her arms.

The judge said McRae approached her from behind and stabbed her in the neck, shoulder and head. When she leaned forward to protect the baby, he stabbed her in the back.

Justice Palmer said the stepmother described McRae's expression as "completely blank", saying he only stopped stabbing her when he cut his own thumb.

McRae then said he was "so sorry", Justice Palmer said.

His stepmother ran out of her house onto the street screaming for help, while McRae dialled 111 from the house.

Justice Palmer said McRae had no previous convictions.

However, the police found a cellphone box in his room that had been stabbed a number of times with the same knife.

The police also found McRae's texts to his girlfriend describing how he had imagined killing his whole family and that when he saw a girl on the street "one part of him wanted to be nice and sweet to her and the other part wondered what her head would look like on a stick", the judge said.

Justice Palmer said McRae had also searched the internet for the best place to stab and kill someone, as well as the best serial killer films.

McRae had no history of drug or alcohol use but was diagnosed with depression. He had three psychiatric reports, which found there was no mental illness and he was fit to stand trial.

The reports found him to be at high risk of re-offending with the potential to cause grievous bodily harm or death.

While the actions of the attack were pre-meditated, the selection of the victim was impulsive, the report said.

One of the most recent reports, in March, described how McRae had "hundreds of homicidal thoughts a day" including thoughts of killing a staff member while he was in custody.

Justice Palmer said the attack was prolonged, unprovoked, gratuitous, and caused serious injuries.

"Your stepmother sustained a punctured lung, fractured rib and cuts to her jaw, neck and face," he told McRae.

"She says in her victim impact statement, she doesn't think she'll ever be the same again... or her family."

He said McRae may be in the early stages of psychosis, which he said could either develop or lessen as he aged.

Justice Palmer said prison was unlikely to be good for McRae but that he had to take into account the risk to the public.

The major mitigating factors were McRae's young age, the fact he was a first-time offender and that he had entered an early guilty plea. The support of his family was also acknowledged.

The sentence was reduced to six years and seven months, with no minimum sentence.

The judge also stressed the importance of McRae receiving proper psychological care while serving out his sentence, after McRae's defence lawyer Clare Bennett told the court that there had been inadequate care at the adult facility.

She said as a youth justice advocate she recommended name suppression but her client rejected it.