ASIO said customers were generally happy with its reporting: "Our analysis of terrorism motivations, influences and trends in Australia was highly regarded, as were our biannual threat assessments of terrorism and violent protest and reports on right and left-wing extremism."

By contrast, the equivalent New Zealand agency's annual report made no mention of right-wing concerns.

The only recent ASIO-reported incidence of right-wing terrorism in Australia was Operation Fortaleza on August 6, 2016, when an extreme right-wing individual's alleged preparations were disrupted. The person was Phillip Michael Galea, 31, of Braybrook in Melbourne's west, who was charged in 2016 with "planning to commit a terrorist act and collecting material in connection to a terrorist act".

The prosecution claimed Galea was targeting the Melbourne Anarchist Club in Northcote, the Resistance Centre in the CBD and Trades Hall in Carlton. Galea apparently wanted to produce what he called The Patriots Cookbook, which would show how to make bombs using potassium nitrate "for the advancement of extreme right-wing ideology to overcome the perceived Islamisation of Australia".

Galea reportedly had links with right-wing extremist organisations such as Britain's Combat 18 and the United Patriots Front in Australia.

In 2017, The Sydney Morning Herald reported on the conviction of "neo-Nazi" Michael James Holt, 26, who had threatened to carry out a mass shooting and considered targeting Westfield Tuggerah on the NSW Central Coast. Holt had made guns, knuckledusters and slingshots in his grandfather's garage. Subsequent raids on his mother's home and a hotel room discovered more weapons.

The deadliest lone-actor right-wing attack in recent times was the July 22, 2011 bombing and shootings in Norway by Anders Behring Breivik, aged 32. He set off a massive vehicle bomb in Oslo outside prime minister Jens Stoltenberg's office, killing eight people, and then went on to Utøya island to shoot and kill 69 young members of the Labour Party who were meeting and holidaying there.

Breivik's preparation and attack have become the blueprint for similar attacks. On the day of the attacks, he distributed electronically a compendium of texts titled 2083: A European Declaration of Independence, describing his militant ideology. His manifesto included details of how he played video games such as World of Warcraft to relax, and Call of Duty: Modern Warfare 2 for "training-simulation".

Breivik bought a semi-automatic 9-millimetre Glock 34 pistol legally by showing he was a member of a pistol club, and a semi-automatic Ruger Mini-14 rifle after buying a hunting license. He told a court he trained for shooting using a holographic device while playing Call of Duty. He said it helped him gain target acquisition.

In 2012, the British home affairs committee warned of the threat of far-right terrorism in Britain, saying it had heard "persuasive evidence" about the potential danger and cited the growth of similar threats across Europe. In March 2018, Mark Rowley, the outgoing head of Britain's counter-terrorist policing, revealed that four far-right terror plots had been foiled since the Westminster attack in March 2017.

In the United States, there have been 18 right-wing terrorist attacks since 2012, resulting in 51 deaths. The deadliest, on October 27, 2018, was a lone-actor right-wing attack by Robert Gregory Bowers, 46, at the Pittsburgh synagogue, where he shot and killed 11 worshippers. It was the most lethal attack on the US Jewish community.

Bowers had earlier posted anti-Semitic comments against the Hebrew Immigrant Aid Society. Shortly before the attack, he posted on "Gab" that "HIAS likes to bring invaders in that kill our people. I can't sit by and watch my people get slaughtered. Screw your optics, I'm going in." (Gab is an ad-free social network "dedicated to preserving individual liberty, the freedom of speech, and the free flow of information on the internet".)

Many of the shooter attacks in the US do not show a clear motivation. The FBI designated 50 shootings in 2016 and 2017 as "active-shooter" incidents. There is no assessment yet of numbers for 2018. Meanwhile, the Gun Violence Archive recorded 152 mass shootings in the US between 1967 and May 2018, averaging eight deaths per incident.

The challenge for security organisations is to identify potential lone-actor right-wing attackers. Some options are monitoring gun clubs and their members, social media, extreme-right organisations and their members – and encouraging anyone with concerns about a potentially violent person to report them to the national security hotline. Effectively, it means using lots of resources for small returns.

Police may pick up on some potentially violent persons through threat assessment centres, and it's important, of course, that police respond quickly to an active-shooter situation. It would also be desirable to have an armed police officer present on days of worship at mosques and synagogues (rather than armed civilian guards), and possibly one or more responsible members of likely target organisations trained to use a gun, with access to a weapon securely stored on-site.

Australia also needs another gun buy-back scheme to reduce the number of firearms in the country (there are now 3 million registered guns plus an estimated 260,000 unregistered.)

Clearly, as Brenton Tarrant showed only too well in Christchurch, the well-organised lone-actor extremist can be a deadly attacker. The law-enforcement challenge is that the perpetrator might not have done anything illegal until the attack begins.

Clive Williams is a visiting professor at the ANU's centre for military and security law.