The now 22-year-old was convicted of murder earlier this year for strangling 18-year-old Emerald Wardle at a home in Metford in 2020, telling police after his arrest he believed she was a "demon" and that either he or she had to die.

In Newcastle Supreme Court on Thursday, Justice Richard Cavanagh, SC, said he accepted that Miller was experiencing delusions brought on by the use of cannabis and LSD when the crime was committed, but "the taking of drugs does not provide any excuse for such violent conduct".

Miller was found guilty in June after he sought to be dealt with under a provision of law which would have allowed a jury to find him guilty but not criminally responsible for Miss Wardle's death.

In delivering their verdict, the jury rejected the defence case that Miller's psychosis was the first episode in an underlying mental illness - such as chronic schizophrenia - and instead was solely drug-induced.

He has not been formally diagnosed with schizophrenia since the murder.

Whether Miller killed Miss Wardle was never in question - he told police he had done so.

Justice Cavanagh said on Thursday "the simple reality of this case and sad tragedy" was that Miss Wardle would not have been murdered if Miller was not a drug user.

He said Miller had acted under a "genuinely held belief" as a result of his psychosis and was not intoxicated by the drugs he had taken days earlier, but was experiencing a temporary period of psychosis.

Miller was described as a relatively long-term drug taker who used cannabis daily and had taken the mind-altering drug LCD about 30 times in his life - the final dose 11 days before he murdered Miss Wardle.

The court heard that Miller and Miss Wardle had been in a romantic relationship for about two years before the attack and that Miller had never previously behaved in a way that would have made Miss Wardle or her family believe she was in danger.

But at a home in Galway Crescent in the early hours of June 20, 2020, Miller killed Ms Wardle in a frenzy believed to have lasted two to three minutes.

While Miss Wardle's cause of death was strangulation, Justice Cavanagh said the teenager had suffered bruising to her torso and a torn small intestine, as well as injuries to her face and scalp - which indicated Miller "did more than just strangle".

On the night of the murder, Miller called Triple Zero and asked to be connected to a mental health counselling line, telling the operator he had killed a person and that she was a demon.

Police arrived and found Ms Wardle's body in the bathroom.

Miller told police after his arrest he killed Ms Wardle and that he was sorry.

The court on Thursday heard passages from a letter Miller penned to Miss Wardle's family expressing remorse, in which he wrote "with every part of my being, I'm so sincerely sorry" and that he was in "a nightmare I can never wake from".

Justice Cavanagh said Miss Wardle was not a drug user, despite the habits of her boyfriend, and had been in a "seemingly good relationship" with Miller.

"Miss Wardle was a person with a strong character and a bright future ahead of her," he said.

Miller was handed a non-parole period of 13 years and his sentence was backdated to June 2020 to account for time he has spent in custody.

He will be first eligible for release in 2033.

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