However, Simon was deemed able to make his own decisions and had refused to co-operate despite numerous attempts to get a psychiatric report.
Justice Katz said at the time of the attack Simon was living with his parents, in the same street as his brother and his family.
In the lead-up to the attack Simon was acting strangely and wasn't sleeping. He was paranoid and his family described his behaviour as "increasingly odd". Simon became convinced his long-term partner and brother were having an affair, the judge said.
She said his parents described him as "being on another planet" and his brother was so concerned he had padlocked his gate.
Justice Katz said the family were becoming increasingly concerned, and the night before the attack, his sister-in-law called 111 several times.
At 5.30am on August 5, while his brother was asleep, Simon kicked in the door and stormed down the hall. He was carrying a knife he had brought with him and when he found his brother asleep he accused him of having had sex with Simon's partner.
Justice Katz said Simon then stabbed his brother in the head and face "with such force the blade broke off and became lodged" in the victim's head.
Simon then went to the kitchen to get another knife while his brother "who could barely see due to the blood" grabbed a machete and hit him in the back of his head, before fleeing.
"Unfortunately he tripped. You stabbed him in the stomach while telling him to die."
The knife blade was still embedded in the victim's head when he ran to the neighbours for help, Justice Katz said.
She said the victim suffered a number of serious wounds, had undergone operations after serious complications related to the abdomen wounds and had spent 10 days in hospital.
She said the brother suffered serious ongoing health issues; the victim's partner suffered emotional trauma and his children were also affected.
"It took place in front of your brother's young children. They are fearful and have ongoing issues as a result."
Justice Katz told Simon even though his brother was the victim, his children's relationship with their father's side of the family was seriously damaged and they had suffered as a result.
"You attacked him in his own home, somewhere he should have been entitled to feel safe."
Crown prosecutor Ngaroma Tahana acknowledged the failed attempts at a psychiatric assessment had left the court in "an awkward position".
However she said a number of factors had to be taken into consideration, including the violence, use of weapon, attack on the victim's head and the home invasion aspect.
She said the effects extended beyond the primary victim and had a significant impact on the wider whanau and the relationships within it.
Simon's lawyer, Moana Dorset, acknowledged the court had "bent over backwards" to attempt to get a psychiatric evaluation.
She said an underlying message through a cultural report was the effect of long-term methamphetamine use and abuse.
Ms Dorset said Simon had family support and was concerned he had never been physically assessed following the machete blow to his head, which required 27 staples.
"He is troubled regarding the effect of this on his family."
Simon, who interrupted the sentencing to say "they've illegally bugged me", asked afterwards "so what happens with the bug?''.
