Justice Brewer said that in the weeks before the killing, there were a number of occasions where Hill had been aggressive towards Melrose.
About 10.30am on June 12 when Hill and the victim were alone together at the home she and his father shared, he became aggressive towards her.
A short time later, she called Hill's father and cried on the phone about Hill's aggression.
Hill then took the phone and said to his father, "she's the one to blame, it's all her fault,
it's getting ugly".
After the phone call ended, there was a physical confrontation outside on the front lawn between Hill and Melrose, and she was not to blame for this.
Hill struck Melrose eight or nine times about the head and face with a hammer causing her to fall to the ground. She died a short time later.
When Hill's father returned to the house he found his partner lying on the lawn with the bloodied hammer nearby and his son was inside the house, having just had a shower.
He and another relative restrained his son until police arrived.
Hill told police he believed Melrose was performing satanic rituals at the house and she had taken control of his mind, placing him in a trance and preventing him from leaving the address.
He admitted he inflicted the fatal blows with the hammer.
A doctor's report commissioned by the Crown said it appeared that Hill had been experiencing symptoms of psychosis for several months before the homicide.
A general practitioner referred him to Community Mental Health Services and prescribed risperidone.
After Hill's arrest, he was seen by the Prison Mental Health Service, where he was diagnosed with psychosis and began antipsychotic medication.
Hill told the doctor he believed he was being controlled by the devil, and also believed animals were staring at him and could sense the devil in him.
He also told the doctor he believed the devil was trying to get him and Satan was going to come from the earth and take him to hell, and that the television was directed at him.
"In particular he believed his father's partner was a witch, could read his thoughts, and was taking over control of his body. He began to hear voices, which became worse after he began medication.
"The voice was often indistinct and spoke in tongues, such that he could not understand what was being said," Hill said.
Two doctors recommended Hill be found not guilty of murder by reason of insanity.
Justice Brewer said he was satisfied on the balance of probabilities that at the time of the offending Hill was labouring "under a disease of the mind" which rendered him incapable of knowing that striking Mrs Melrose with the hammer was morally wrong.
Justice Brewer said Hill had been diagnosed with schizophrenia and had made good
progress with the treatment he was receiving.
But Hill's current condition, when considered against his history of mental health problems, meant he posed a "serious risk" to the health and safety of others.
Justice Brewer ordered Hill to be detained in a hospital as a special patient under the Mental Health Act.
