Australia’s terror threat has been lowered from probable to possible but security heads are warning the likelihood of an attack is 50-50.

The defeat of the Islamic State and an ineffective al Qaeda propaganda machine failing to connect with Western youth has resulted in fewer extremists in Australia.

The greatest threat to Australians remains a lone wolf attack from someone radicalised quickly and using an easily accessible weapon such as a knife or car.

ASIO director-general Mike Burgess said while the threat level had been lowered for the first time since 2014, it did not mean the risk had been eradicated.

“This does not mean the threat is extinguished,” he said on Monday.

“It remains plausible that someone will die at the hands of a terrorist in Australia within the next 12 months.”

There has been an increase in radical nationalism and right-wing extremist ideology in the past couple of years.

“Individuals are still fantasising about killing other Australians, still spouting their hateful ideologies in chat rooms, still honing their capabilities by researching bomb-making and training with weapons,” Mr Burgess said.

Since 2014, there have been 11 terrorist attacks and a further 21 plots have been disrupted. Half of the foiled plots were in the first two years, when IS was more prominent.

There have also been 153 terrorism-related charges stemming from 79 counter-terrorism operations since 2014.

Mr Burgess warned it was almost guaranteed that the threat level would increase again.

But this would not necessarily be off the back of a terrorist attack, with the overall security assessment taking into account lone attackers.

People are being radicalised online at an extreme pace, sometimes in as short as weeks or months.

But there are fewer groups planning months or years-long sophisticated terrorist attacks with the aim of maximum destruction.

More than 50 people convicted of terrorist offences are also due for release in the future. Only a small number will be freed by 2025.

There had also been increases in COVID-19 grievances and hateful rhetoric online, but this was unlikely to lead to violent actions, Mr Burgess said.

“We must distinguish between ugly actions, big talk and actual terrorism,” he said.

“While some individuals used violent rhetoric and some protests involved violence, we did not identify acts of terrorism,” he said of COVID-19 conspiracy theorists.

Another senior security official described it as “awful but lawful”.

The assessment also considered the risks posed by the four adults and 13 children recently repatriated from a Syrian refugee camp to western Sydney

Mr Burgess said all had been assessed to be low risk.

-AAP