A Perth man was acting upon "command hallucinations" and not in control of his actions when he fatally stabbed his mother, eight-year-old brother and 15-year-old sister, a court has been told.
Teancum Vernon Petersen-Crofts, 21, is facing a judge-alone trial in the WA Supreme Court over the deaths of his mother Michelle Petersen, 48, eight-year-old brother Rua and 15-year-old sister Bella at their Ellenbrook home on July 15, 2018.
He has pleaded not guilty on unsoundness of mind to murdering the trio.
The trial has heard Petersen-Crofts turned up at his local police station on July 13, shirtless and sweating profusely and claiming his neighbour was a serial killer.
He was detained and transferred to the emergency department at St John of God Hospital in Midland, where doctors said he was likely floridly psychotic.
But the following morning he appeared calm and doctors agreed to discharge him, despite his mother's concerns.
Forensic psychiatrist Daniel de Klerk on Monday told the court Petersen-Crofts had been admitted as an inpatient with psychotic symptoms on 12 occasions and presented at emergency departments many more times.
He said he was surprised Petersen-Crofts had not "been taken more seriously" by doctors at St John of God Hospital.
"The outcome could have been very different had he been admitted," Dr de Klerk said.
Petersen-Crofts, who has been diagnosed with treatment-resistant schizophrenia, had experienced "command" auditory hallucinations from various deities and believed his family were plotting to kill him, Dr de Klerk said.
He believed that while Petersen-Crofts knew what he was doing, he did not have control of his actions or know that he ought not to do the act.
"When one's god orders you to do something it is very difficult to resist, and especially if the act is to preserve your own life," he said.
Psychiatrist Steve Patchett agreed that Petersen-Crofts likely didn't have control of his actions, noting that he had displayed symptoms of a psychotic disorder since the age of 14 when first admitted to hospital in his native New Zealand.
In a recorded police interview the day after the alleged murders, Petersen-Crofts claimed his neighbour was a serial killer and blamed him for killing his family members.
Later in the interview, he said a deity named "Big Papa" had committed the crimes.
He also claimed he had "saved four billion people last night" and repeatedly stated "put me on death row" before refusing to answer further questions.
The court has heard each of the victims received between 44 and 54 stab wounds from a large kitchen knife, while neighbours had reported hearing screams and what one person described as sounding like a "growling animal".
Prosecutor Paul Usher said Bella was still alive in the backyard when police arrived and that paramedics had reported her softly pleading for help.
She died in an ambulance on the way to hospital.
CCTV footage of a shirtless Petersen-Crofts pacing outside a nearby 7-Eleven service station early on July 15 and then speaking to an employee through a service window was also played, as was a recording of a triple-zero call made by the worker.
Mr Usher said Petersen-Crofts, who had blood on his hands, told the employee he had just killed his mother and his siblings.
Ms Petersen had been attempting to get help for her son and had told friends she feared for herself and her younger children because of his increasingly bizarre behaviour.
Petersen-Crofts has been detained in a secure psychiatric facility since his arrest and is being supported by family members.
The trial is expected to conclude on Monday.
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