After the Paris and Brussels attacks, Australian authorities delivered the reassuring message that three key differences distinguished our own threat situation. Our extremist networks, while serious and enduring, are relatively small compared with Europe's; our island nation gives us a strong advantage in controlling movement across our borders; and our strict gun laws make it hard to get hold of powerful firearms.

The final of these strengths is a critical defence that keeps Australians safer and is consistently highlighted by national security officials. But it doesn't apply when an attacker can drive a large lorry into a packed crowd. Vehicles have formed part of attacks or plots before, but usually as enablers - running down a police officer and taking his gun to then use on others for instance.

But this lies on another scale and it represents a shift in tactics. No gun is needed when you can use as damaging a weapon as a large truck travelling at 70 kilometres per hour against a large number of humans.

This approach has evolved quickly among Islamist networks since 2014 when Islamic State spokesman Abu Muhammad al-Adnani urged followers around the world to use whatever weapons were at their disposal to carry out attacks.

If would-be jihadists could not obtain a bomb or a gun to kill their victims, they should "smash his head with a rock, or slaughter him with a knife, or run him over with your car, or throw him down from a high place, or choke him or poison him", Adnani said.

This is a leap in that evolution. It shows us that there are loopholes in Australia's relative advantage.