The man who killed six people and injured countless others at a Bondi shopping centre on Saturday, 40-year-old Joel Cauchi, reportedly had "mental health issues", police explained soon after the tragic event, while ruling out terrorism.

However it's important to note the vast majority of people with mental illness do not pose a risk of violence to others.

Tragically, there is still an unacceptable level of stigma and misunderstanding of mental illness, including the mistaken belief that people with mental illness are violent. People may draw conclusions from cases such as the Bondi attack, where people with histories of mental illness engage in violence.

So is there a link between mental illness and violent crime? Here's what the evidence says.

Research from Australia and overseas shows a small percentage of people with serious forms of mental illness may be at increased risk for violence.

Our research in Victoria, for example, shows 10 per cent of people with schizophrenia (a serious form of mental illness where the person can be so unwell as to be out of touch with reality) have perpetrated a violent crime. This compares with about 2.4 per cent of the general population. So, while the people who have schizophrenia were more likely to have a violent offence, the vast majority of them did not.

The findings are mixed regarding a direct relationship between more common mental illnesses, such as anxiety and depression, and violence.

Although the reasons that anyone - including people with psychiatric illnesses - offends vary, we identify three categories of people with mental illness who engage in violence.

The first is the very small group of people with a serious mental illness, typically schizophrenia, who act violently as a direct result of symptoms of mental illness.

For these people, their illness leads to irrational thinking and beliefs that can increase the likelihood of them behaving violently. A person may develop delusional beliefs they are being targeted or their lives are in danger if they do not act violently against perceived enemies.

For these people, if they did not have the particular symptoms of mental illness, they would not offend.

People in this category may be found not guilty by reason of mental impairment. They are then typically held in secure hospitals or prisons where they are treated and eventually released, when they are no longer found to be a risk to others.

The second category is much larger, and more varied. For this group, people do not offend because of their mental illnesses, per se, but due to the related individual and social issues that may accompany mental illness.

People with some forms of mental illness may be more likely to engage in substance misuse, for example, which may, in turn, contribute to offending.

Many of the negative social factors associated with serious forms of mental illness overlap with the negative social factors that increase the probability of being violent.

People with serious forms of mental illness who have backgrounds characterised by social and family disruption and disadvantage together with abuse, behavioural disturbances, substance use and educational failure and disengagement are significantly more likely to offend than people with mental illnesses who do not have such disturbances in their backgrounds.

Of course, most people with a psychotic illness do not come from such disadvantaged backgrounds.

Research and clinical experience also show factors related to offending within this group are similar to those who do not have mental illness. In addition to substance abuse, this can include violent attitudes, exposure to trauma and violence, association with people who are antisocial, and poor family and professional support.

The final group of people with mental illness who offend do so irrespective of their mental illness. People in this group are typically characterised by early onset antisocial and illegal behaviour.

They differ from other offenders with mental illness by having a pervasive and stable pattern of offending regardless of their mental state. This behaviour almost always precedes the onset of mental illness.

While people with a psychopathic or antisocial personality disorder will be included in this group, not all of the people in the group will have such a personality disorder.

It's not the mental illness per se that causes people to be violent. Rather, it's symptoms of the illnesses and related factors.

There is good evidence therefore that providing psychiatric and psychological care can help manage symptoms of mental illness and reduce the likelihood of violence.

It's also important to address the broader factors that are related to offending and violence among people who are mentally ill.

Unfortunately, partly as a result of the pressures on mental health services, staff have few resources to help address the array of factors that can lead to one behaving violently. Continued investment and education is required to boost the services and address the factors that lead to violence among people with mental illness.

While we have made some progress in the recognition that mental illness affects a large percentage of the population, individual acts of violence committed by someone with mental illness must not lead us to jump to conclusions that all people with mental illness are violent.

Support is available for those who may be distressed. Phone Lifeline 13 11 14; Men's Referral Service 1300 776 491; beyondblue 1300 224 636; 1800-RESPECT 1800 737 732.