Over the past five years or so, the likelihood of an Islamist attack in Australia has been high but the consequences probably low in terms of casualties; by contrast, the likelihood of a far-right terrorist attack had been low, but the consequences probably far higher in terms of casualties.

Why is there a difference in lethality?

Islamic State has superseded al-Qaeda as the dominant force in global Islamist terrorism over the past five years and its focus has been on "low-tech, high-impact" attacks - mainly vehicle rammings and knifings. Islamic State has preferred the kind of basic attack that any unsophisticated sympathiser could undertake with little or no preparation.

By contrast, al-Qaeda's emphasis has always been on well-planned mass-casualty bombings and shootings. Al-Qaeda's most dangerous legacy (although the organisation is by no means down and out) was a series of articles in its Inspire magazines on issues like how to make a bomb in the kitchen of your "mom".

Some Islamic State supporters also read Inspire and that perhaps could have "inspired" the plan to down an Etihad plane out of Sydney in June, 2017. We therefore can't totally discount an Islamist mass-casualty attack in Australia.

The far right, on the other hand, tends to be inspired by its violent idols - foremost among them being Timothy McVeigh in the US, Anders Behring Breivik in Norway, and Brenton Tarrant in New Zealand. All three used bombings and/or shootings to cause mass casualties.

Former soldier McVeigh bombed the federal building in Oklahoma City in 1995, killing 168 people. It was primarily an act of revenge against the federal government for the Waco "massacre". McVeigh did not formally belong to any extremist group. He chose not to appeal against his execution to remain in control of his fate. His favourite poem was Invictus: "It matters not how strait the gate, How charged with punishments the scroll, I am the master of my fate, I am the captain of my soul."

Breivik killed eight people in 2011 by detonating a vehicle bomb in Oslo, and then shot dead 69 participants at a Workers' Youth League summer camp on Utoya Island. Breivik identified himself as "a fascist and a Nazi, who practices Odinism and uses counterjihadist rhetoric to support ethno-nationalists".

Australian Tarrant undertook shooting attacks at mosques in Christchurch, New Zealand in March, 2019, killing 51 people. Tarrant is a white supremacist and part of the "alt-right", but apparently not an active group member. He had, however, visited groups in Europe.

Extremists inspired by Tarrant praised him as a "saint" online, and subsequently committed attacks of their own in Poway, El Paso and Norway.

Concern is often expressed about the threatening activities of Australian far-right groups. The reality is that such groups are long on rhetoric and thuggish behaviour, but in the past have been more likely to kill their own members than members of the public.

"National Action" killed two of their members suspected of being police informers for that reason. Unfortunately for them, ASIO was bugging the premises where one of the murders took place - and took the ethical decision of providing evidence that led to the perpetrators' conviction.

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Australian far-right groups now include Nationalist Australian Alternative, Lads Society, Antipodean Resistance, Proud Boys, Soldiers of Odin, Identity Australia, Australian Traditional, New National Action, Patriotic Youth League, Rise Up Australia, Yellow Vest Australia and so on. Some are short-lived (and may already have disbanded), and most don't have many members, but they can generate large numbers of supporters when they are able to hijack an incident - as they did during the 2005 Cronulla riots.

While most Australian far-right groups are white nationalist, they may also embrace other right-wing ideologies such as fascism and neo-Nazism. Sometimes they share popular national concerns. For example, many Australians probably believe that an immigration figure of 200,000 a year is not sustainable for environmental and social reasons. Far-right groups oppose it for nationalist or racist reasons - notably because most of the migrants are from Asia.

In fact, in recent years most far-right terrorist attacks have not been by carried out by groups but by adult males who are not active members. They have absorbed far-right ideology, usually related to white nationalism, antisemitism, and Islamophobia. They have made contact and met with like-minded individuals through internet chat sites.

These lone actors are a difficult intelligence target. ASIO penetration of far-right groups will not necessarily turn them up. The most likely way to identify potential problem individuals is through their communications (usually encrypted), membership of gun and hunting clubs, the kind of literature they read, and links to bikie gangs.