The random cruelty of the crime releases a wave of shock, anxiety, sympathy and then a search for reasons. Why do such events occur? Why does someone deliberately harm innocent strangers?

Badness, madness, or both. Ideological driven terrorism is sadly one all too familiar explanation, but this seems to have been ruled out this time. While specific knowledge of the background of the perpetrator remains unclear in this case, random multiple homicides of this kind always raise questions about the role undiagnosed and untreated mental illness may have played.

The relationship between violence, homicide and mental illness is complex and has not been well understood in the public mind.

The film industry has created the indelible impression that there is a strong link between psychotic illnesses, especially schizophrenia, violence and homicide.

This most unfairly has fuelled stigma and added to the huge burden borne by people with serious mental illness and their families.

The vast majority of people with serious mental illnesses are not violent or homicidal. However, there is nevertheless a subgroup of people with psychotic illnesses in particular, who if untreated or poorly treated, are indeed more likely to commit homicide.

This is an inconvenient truth, which has been a challenge for the mental health field, but it must be faced, because there are ready solutions at hand which have been neglected.

The small subgroup of people with psychotic illness who commit murder is more likely to be young and male (the peak group for the onset of all offending) who are also likely to be abusing substances, have a history of other offences, and to have evidence of personality dysfunction stemming from social adversity.

The good news is that these homicides are by no means inevitable. Sixty per cent of homicides committed by psychotic patients occurred before they were ever diagnosed and treated, and the remainder by poorly treated patients.

Treatment is very effective in a first episode of psychosis, and effective treatment restores the level of risk to normal, provided it is maintained.

The public safety benefits from specialised early detection and intervention programs for young people with emerging psychosis, along with reduced suicide rates and better employment outcomes, are one of the key reasons why these programs have been scaled up around the world, based on the Victorian EPPIC (Early Psychosis Prevention and Intervention Centre) model (an approach still not available to all Victorians).

Across the lifespan, early intervention and rapid access to care for relapsing patients would reduce the risk to the community. Homicide cannot be accurately predicted, so easy access to care for all with serious mental illness is the only safe approach.

In the 1990s, with many more seriously mentally ill now in the community, real risks to discharged patients as well as the community led to a joint venture between Victoria Police and the mental health system known as "Project Beacon".

This involved CATT teams and police attending crises in the community together. This proved to be very effective and should be revived and expanded. We have been involved in training Victoria Police to respond safely to young offenders with mental illness, and in general they do a remarkable job.

But they are placed in untenable situations and need expert help at the scene, not armchair advice.

The fundamental problem is that the capacity of the Victorian public clinical mental health system has been declining for well over a decade and is overwhelmed.

The mainstreaming of mental health care in the 1990s, with a major reduction of beds, depended totally on a dynamic, and expanding system of expert multidisciplinary community mental health care being built to reduce extreme and desperate acute presentations to hospital. This began well but has been largely abandoned.

Victoria has been characterised as the Argentina of mental health care in this country, having once invested more per capita in mental health care than any other state, it now languishes at the bottom. Public mental health services particularly in the growth corridors have retreated from community care, overwhelmed by acute demand.

Safety is core business for a state government. Whether or not Friday's disaster turns out to be linked to failure to detect and treat or not, these tragedies are happening writ smaller every year now.

In the northwest of Melbourne we have seen population growth of 50 per cent in the past decade and yet no more resources for mental health. Previously in our services suicides were rare. Now they are a regular occurrence. And for the first time we have seen a spate of homicides committed by patients who could not get timely access to sufficient expert care.

All these are highly preventable deaths and represent a barometer of public safety. The Andrews government, to its credit took office promising to deal with this serious decline and has not swept the issue under the carpet.

The 2016 government report by Stephen Duckett into hospital safety revealed the sheer scale of under-investment in clinical mental health care, and again to the government's credit this was welcomed by Ministers Foley and Hennessy, who have promised to really fix it this time. Window-dressing and Band-aids will not work this time. Major redesign and investment and integration with primary care and third sector programs is urgent and essential.

The solutions are clear - a complete redesign and rebuild of the whole public clinical mental health system especially the mobile outreach elements which must be in proportion to the population growth.

The creation of a world class Youth Forensic Mental Health model of care.

Young people under 25 have the highest risk of offending behaviour and of mental illness and there is substantial overlap. There is enormous potential for prevention and improvements in public safety in addressing the needs of this group. Victoria has never had a specialised forensic mental health unit for young offenders. Some of the recent crisis in the Youth Justice is partly a consequence of this policy failure.

We may never reduce the risks of homicides from this source to zero, however if we do not address the yawning gaps that are now fully acknowledged by government, then more people will die unnecessarily in the coming years through preventable deaths.

Professor Patrick McGorry AO

Professor of Youth Mental Health University of Melbourne

Executive Director Orygen, the National Centre of Excellence in Youth Mental Health

Associate Professor Rosemary Purcell

Director of Research

Orygen, the National Centre of Excellence in Youth Mental Health