The 24-year-old musician had just come from a band rehearsal and was on his way to meet a friend who had just finished performing at a hotel.

In the next set of seats at the bus shelter sat Geoffrey Webber, a 25-year-old with paranoid schizophrenia.

Without warning or even a hint of provocation, Webber leapt up, brandished a 30cm, black-handled carving knife and, went for the younger man.

"Their arms were above their heads and [Webber] had the knife up and [Luke] was holding his hands up … trying to stop the knife from coming down," one witness, Vicky Thorpe would later tell police of the ensuing struggle.

As onlookers frantically called triple-0 and yelled at Webber to stop, he stabbed Luke twice in the chest causing severe injures that would cause his death soon after.

On Thursday, more than three years after Luke's funeral, Justice Stephen Campbell found Webber not guilty of murder on the grounds of mental illness, declaring that the 23-year-old had been "an entirely innocent victim of a brutal and frenzied attack born of a diseased and deluded mind".

The judgment reveals that Webber had a long history of mental illness and engagement with local mental health services stretching back 10 years. Nevertheless, at the time he killed Luke, Webber had received no treatment for paranoid schizophrenia, and was not under any form of formal supervision.

Webber first came to the attention of the public system at the age of 15 when he was forced to live in refuges after his schizophrenic father left his mother, who suffers from bipolar disorder.

A therapist described Webber as "isolated, reclusive [and] avoidant" and diagnosed him with a depressive condition.

Webber's first documented psychotic episode occurred in December 2008 at Manly hospital when he expressed the belief that a spy plane was flying outside the hospital window.

He again came into contact with mental health services in 2010 after becoming aggressive with a neighbour whose balcony was supposedly dripping water onto his washing.

In April 2011 police were called to the young man's unit after he set up a web camera above his front door and repeatedly told another neighbour "they're coming for me". He voluntarily attended Manly hospital who released him soon after.

In 2012 Webber began to exhibit clear signs that he posed a risk to the community, in what Justice Campbell described as "a series of very worrying incidents leading up to him killing Mr Robins".

Within the space of two days in April, he assaulted an elderly resident at his unit block, followed an elderly man back to his home and intimidated a young woman who was pushing her child in a stroller.

On April 30, in response to a call from Webber's mother, two nurses from the Mental Health Crisis Team at Manly hospital visited the young man at her home.

Webber refused to speak to them and they suggested that Mrs Webber try to persuade her son to attend the hospital and told the police about their concerns.

Mrs Webber could not bring herself to confront her son, and the police either did not or could not contact him.

Webber remained free to come and go as he pleased. Two days later he sat, armed with a knife, at a northern beaches bus stop.

"All of this evidence ... paints a compelling picture ... of a genetically predisposed, socially and emotionally deprived young man, descending into the grip of a serious mental illness at an ever quickening pace," Justice Campbell said.

"In my view ... he was probably suffering from that disease at least since his admission to Manly hospital in December 2008. As the disease went untreated it probably steadily worsened over the years."

He ordered that Webber be "detained in a correctional facility or at such other place as determined by the Mental Health Review Tribunal until released by due process of law".

A spokeswoman for the Northern Sydney Local Health District said that an "external review conducted in March 2014 indicated that no specific recommendations arose from the care of Mr Webber".

She said that Manly hospital had sufficient resources to treat people with serious mental illness.