Justice Fiona Seaward agreed, finding that on the balance of probabilities, it was likely Mr Thomas was mentally impaired when he took his father's life.

In her judgment handed down last week, Justice Seaward wrote Mr Thomas cannot be considered criminally responsible and must be found not guilty, but ordered he remain in custody on a limiting term of life imprisonment.

The decision also revealed that about an hour after Mr Thomas killed his father he washed the blood from his hands, had a drink and a smoke, and went to a local police station.

"I have just killed my dad," he told the policeman.

The officer asked Mr Thomas how he'd done it, to which he replied: "I've hit my dad with a frying pan and gouged his eyes out and slit his throat".

Police found Mr Thomas's father's body in a pool of blood in his lounge room with a steak knife beside him. One of his eyeballs was found in his bedroom, the other by a sliding door.

Mr Thomas told investigators he struck his father three times with the frying pan before grabbing him around the neck and choking him.

"I was trying to end his life and it got to be quite a struggle so I've used my fingers to gauge his eyes to try and take out his sight," he said.

"He was quite strong and after that I picked up a knife and slit his throat and held his hand, he squeezed it and he left the world."

Mr Thomas told detectives his father had been annoying him.

"He wasn't exactly a terrible man. We never really got along but we weren't complete enemies," he said in a police interview.

"He was just getting older and older and older."

The court documents also stated Mr Thomas had experienced mental health issues since he was a teenager, and had previously been violent and aggressive with family members.

He also refused to take prescribed medications, was a heavy drinker, and used cannabis and methylamphetamine.

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