But there is an important difference between what is terrifying and what amounts to terrorism.

In this instance, the attacker appears to be a lone man, apparently with mental health issues. Police have stated there does not appear to be any link to terrorism, despite the fact he yelled "Allahu Akbar". Of course, all of these considerations will become matters for the courts.

The public is understandably concerned. The footage of the pursuit and capture of the knife-wielding man, distributed widely, is disturbing.

But it is important to differentiate random acts of violence from terrorism. Understanding that difference should go some way to reassuring the public.

Terrorist acts are planned and carried out by individuals or groups in support of an extreme religious or political view. They are aimed at intimidating the public while gaining as much publicity as possible for the cause.

Tuesday's tragedy, now the subject of a comprehensive police investigation, appears to be a somewhat aimless attack by a deeply troubled individual.

There will always be troubled individuals among us. It is worthy of discussion as to how those individuals are to be treated.

What can be done to predict and prevent attacks by deranged people? These events are mercifully rare. That, in itself, can make identification and notification of potentially violent actors difficult.

As a matter of policy, governments have determined that mentally ill people - by and large - fare better when they live in the community rather than in institutions.

Historically in NSW, the 1983 Richmond report emerged from the Inquiry into Health Services for the Psychiatrically Ill and Developmentally Disabled. It recommended that the majority of mental health patients should be housed among the community.

Many people previously confined to institutions were released; others who might have been were never so confined.

It was well intended. But there has certainly been anecdotal evidence, at least, of unintended consequences. People who had never previously been in trouble with police came under notice.

We need a fresh discussion, and a formal review, of cases of violence - however isolated - in the decades since the Richmond report recommendations came into effect.

Crucially, it must include the input of experienced police and emergency services who are the first responders (apart from any immediate bystanders, as happened on Tuesday).

From that should flow some informed decision-making on dealing with mental health patients who present a risk to the public. This review is long overdue.

This is not to say that mentally ill people, by and large, are dangerous. They certainly are not. It is not to say they should be institutionalised.

It is to acknowledge that we need to do better when it comes to dealing with those rare cases who may present a danger.

The members of the public who intervened on Tuesday, in the absence of police, and managed to overpower and restrain the man armed with a knife, have rightly been praised for their bravery. There is no doubt their actions saved lives. It was critical until police arrived.

There should also be a distinction between their intervention and that of the "trolley man" who acted in the case of the Bourke Street attack in Melbourne last year.

His attempt to use a trolley to tackle the attacker only served to hinder the police who were already present. He got in their way.

In those six minutes of mayhem in Sydney on Tuesday, it was no time to wait for police. Together, the men brought the knifeman to the ground.

A terrorist may not have been so readily brought down. Terrorist acts are usually committed by people pursuing a cause, misguided as it is, and often they are prepared to die for that cause. Often, thankfully, a lone, deranged man will not have the same level of commitment.

The public should take some comfort from the fact that as terrifying and tragic as Tuesday's attack was, it appears to be the random act of a disturbed individual. It will be dealt with appropriately by police, the courts and other authorities.