Among our national security experts are those who say it is a case of when, not if, terror will strike New Zealand.

The attack in New York - like the vehicle-style attacks in France and Spain - shows how simple and unsophisticated deadly violence can be to qualify as "terrorism".

The New York killer had no firearms beyond an air pistol and paintball gun, knives and a rental vehicle.

He was radicalised domestically - likely predominantly online - and is believed to have operated alone.

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In return, the failed so-called caliphate in whose name he attacked - ISIS - has received worldwide attention for days since he drove a rented vehicle into person after person.

That global attention has been fuelled by social media which provided initial reports and imagery and then propagated by reportage from our free and open Western media.

ISIS cannot lose. Once it has fanned the angry embers of a frustrated and isolated individual to flame, it either has a martyr or a surviving public relations machine with a profile it would struggle to manufacture itself.

The machine which fanned those flames is anonymous and widespread among a global community of extremists. It drives an ongoing propaganda and recruitment drive which sees ISIS' online religious commissars seeding the internet with hate, looking for fertile ground.

It is clear to those who guard our national security that there are risk factors in our society that could provide fertile ground.

The issue has been discussed by the Strategic Risk and Resilience Panel, the future-focused think tank of big thinkers from the public service and private sector formed under the previous government.

The panel, which meets through the Department of Prime Minister and Cabinet, cited "threat of radicalisation of disaffected youth" as a primary concern in discussions about terrorism.

The way to fight such a threat was no quick fix. It suggested "community cohesion" as the best weapon and - given the gaps in our communities - that solution is a generational issue.

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Down in the engine room of national security, those "disaffected youth" have also been a focus.

There are concerns about young people from our most vulnerable communities, who see little of promise in the future and carry anger over their short pasts, are very much on the threat radar.

These are the people who sit at computers, projecting anger across the internet and seeking out those who feel the same way.

These are the people ISIS is appealing to, with its online seeding of violence as a reaction to the angry, the disenfranchised, the hopeless, the lost.

These are the people who are targets for the material ISIS produces - detailed instructions on how to carry out attacks, where to do it and why it should be done.

These are many of the people the NZ Security Intelligence Service guards against and - in many ways - these are the hardest to catch.

In terms of weapon development, ISIS expends little other than a constant, global campaign with the hope of encouraging another attack.

The damage which is done goes beyond the cost of human lives. It is an attack on the public feeling of security and wellbeing, with the assault magnified in impact by its communication across social and traditional media.

New York, which suffered through the extraordinary attack which was 9/11, saw eight people killed and others maimed this week. ISIS expended little but won worldwide attention.

For New Zealand, close partnerships with other Five Eyes nations, and with the Government Communications Security Bureau, are one form of fighting this breed of terror.

Another is the relationship developed by the NZSIS with those in communities which may appeal to these almost-exclusively young men. That includes some youth groups, some mosques and some jails.

And so they watch, and wait, and occasionally, when it seems threat is about to become real, they with police will intercede. They do so with a quiet word, warnings and occasionally - because of the risk publicity poses to their work - with prosecution.

The welcome ceremony for the All Blacks after the Rugby World Cup was a point of particular concern for the security services, as was the America's Cup victory parade.

These might have been key moments of concern but the threat is constant. All it takes is losing track of (or not identifying) one angry young man.

The most likely scenario is a crowded city street and an act of violence which takes only seconds, followed by the armed police response. It doesn't require the assailant to be heavily armed - a knife or a vehicle will do, as we have seen.

There was one angry young man who came to the attention of the NZSIS a few years ago after he posed on Facebook with an imitation automatic firearm. The prospect of such a young man brandishing a BB gun in the wake of a knife or vehicle attack on crowded Queen St would be realisation of a nightmare scenario.

Once that person is committed, it takes little to make international headlines. Most adult New Zealanders carry mobile phones, and an act of violence in a crowded place almost guarantees one of those people will visually capture it and post it online.

Once it hits the internet, that act of violence becomes an act of terror. It is not about the death toll - the brutal slaying of British soldier Lee Rigby in London is one of the West's most infamous scars from terror.

Instead, it is about the assault on all of us, our security and the freedoms sacrificed for precautions aimed at preventing such an attack from happening again.

For the adversary, it is about publicity, the message it sends to other angry, disenfranchised young people and the damage it does to our freedoms.

When that happens here - and the security services have prepared for "when" - New Zealand will take its place on the world stage.