It’s relevant that Cauchi had moved from Queensland because it is common for patients to slip interstate in an attempt to shake off coercive treatment orders. These orders allow police to intervene if patients are not taking their medication, usually in the form of injections.

If he did not have an official address it is likely he was invisible to authorities, including mental health services. Queensland health authorities have confirmed that while there was a past diagnosis of schizophrenia it had been several years since he had had formal contact with their services.

Homicides have continued to drop in NSW as they have across Australia, by just over a half in the past 40 years.

As is common in such cases, he was on the fringes of society in all respects: intermittent contact with authorities, estranged from his family, without a clear address and having a peculiar fascination with knives. It is not yet confirmed whether anyone had taken up his advertisements offering sex worker services. What is especially unnerving is that he does not seem especially deranged in the CCTV footage of the killings, almost nonchalant.

In one of the largest reviews of homicides and mental illness, my colleagues Dr Olav Nielssen, Dr Matthew Large and researcher Georgia Lyons studied records from 1993 and 2016. Homicides have continued to drop in NSW as they have across Australia, by just over a half in the past 40 years. This is confirmed by criminologists Don Weatherburn and Sara Rahman in their 2021 book The Vanishing Criminal.

They cite that crime statistics dropped overall from 2001, and by 2018, “rates of the most common forms of crime had fallen between 40 and 80 per cent and were lower than they’d been in 20 or in some cases 30 years”.

The diagnosis of schizophrenia, as Cauchi has been identified as suffering with, is most associated with serious violence, along with severe personality disorder. The increased risk is genuine but modest, not more than 20 per cent. Such people tend to be in states of psychosis, where they hold unshakeable beliefs divorced from reality.

Warning signs

However, the risk explodes when serious mental illness is associated with drug use, especially amphetamines. The risk of a serious act of violence, be it a lone killing or attack on a public figure, then doubles. Unfortunately, this may well have been the case with Cauchi, where past use of amphetamines and psychedelic drugs have been reported.

The same review confirmed that 38 per cent of attacks were committed with a knife and only 11 per cent with a gun, another confirmation of the success of our gun laws in response to the Port Arthur massacre.

Only six days before Bondi Junction, Cauchi posted on social media a photo of himself holding a surfboard with an invitation to join him. This does not appear to be someone on the verge of a murderous rampage.

Extensive research suggests whether or not mental illness is present, there are inevitably warning signs in the lead-up such as changes in demeanour, uncharacteristic arguments, or sharing of plans – a phenomenon in psychological parlance known as “leakage”. Whether rational or not, people always feel some form of justification.

Terrorism mingles personal resentments with political ideologies. In mental illness it can often overlap with paranoia or commands experienced as voices. I am of course speculating, but him being dressed in the jersey of the national rugby league team suggests to me that he may have thought he was engaging in an act of patriotism.

The preponderance of female victims is most likely linked to them being weaker targets. There may have been a critical life event between his social media post, and the fateful period on Saturday afternoon.

In the midst of it all, a city and nation is in shock, mourning and all the while scratching our collective heads. We can take solace in our shared humanity and the collective goodness clearly on show in the face of barbarity.