The nation's chief security agency says this type of violence has joined spying as its main concern, with the threat predominantly coming from young people radicalised online.
While the level has not been raised due to one specific incident, it's understood ASIO has investigated eight incidents in Australia for alleged terrorism or possible terrorist links.
The Middle East conflict is also understood to have exacerbated the concerns of security agencies about politically motivated violence, although it is not the direct cause of the threat level rising.
Australia's national terrorism threat level is PROBABLE. Director-General of Security — ASIO (@ASIOGovAu) @MikePBurgess says more Australians are embracing extreme ideologies & willing to use violence to advance their cause. Director-General's statement: https://t.co/srLaIByGTl pic.twitter.com/zqetZ7PCAGAugust 5, 2024
It's the first time the level has been raised since 2014, during the height of threats from terrorist group Islamic State.
Probable is the third-highest threat level on the federal government's advisory scale.
It means there is a greater than 50 per cent chance of an onshore attack or attack being planned in the next 12 months.
Prime Minister Anthony Albanese said the national security committee met on Monday morning to discuss the change.
"Probable does not mean inevitable and it does not mean there is intelligence about an imminent threat or danger, but the advice that we've received is that more Australians are embracing a more diverse range of extreme ideologies," he told reporters in Canberra.
"Governments around the world are concerned about youth radicalisation, online radicalisation and the rise of new mixed ideologies."
Mike Burgess says political polarisation and intolerance is increasing across the world. (Lukas Coch/AAP PHOTOS)
ASIO director-general Mike Burgess said more Australians were embracing extreme ideologies which use violence to achieve their cause.
"Politically motivated violence now joins espionage and foreign interference as our principal security concerns," he said.
"Unfortunately, here and overseas, we are seeing spikes in political polarisation and intolerance and civil debate and un-peaceful protests."
Among the eight incidents ASIO viewed as terror-related was the Wakeley church stabbing in April, where two Christian clerics were injured in a stabbing attack during a service in suburban Sydney.
A 16-year-old boy has been charged with a terrorism offence over the attack, which police alleged was religiously motivated.
A teenager faces a terror charge after the Sydney church attack. (Bianca De Marchi/AAP PHOTOS)
Mr Burgess said anti-authority beliefs were also growing, along with a decline in trust in democratic institutions.
"This trend increased during COVID, gained further momentum after the terrorist attacks in Israel and accelerated during Israel's military response," he said.
"The (Middle East) conflict has fuelled grievances, promoted protest, exacerbated division undermined social cohesion and elevated intolerance."
There were also concerns of multiple extremist beliefs being combined to create hybrid ideologies.
The director-general said lone actors were the most likely style of terror attack, often using rudimentary weapons in a crowded or public place.
Mr Burgess also noted a large number of young people were being radicalised.
In some cases, the youngest perpetrator of alleged terror offences was 14, while the eldest was 21.
"Extremist ideologies, conspiracies, misinformation are flourishing in the online ecosystem and young Australians are particularly vulnerable," Mr Burgess said.
The prime minister said social media was continuing to push extremists views on young people.
"One of the things about social media is that the algorithms push people towards more extremes, and they reinforce views," he said.
"If they're logging on for a particular view, it will continue to put that forward, as if that is the only view, and it will reinforce prejudices which are there."
Australian Strategic Policy Institute executive director Justin Bassi said the terror level rise was not surprising.
"Instead of the current global instability that we have - whether it be war in Europe, war in the Middle East - likely to dissipate over the next year or two, it is only likely to become more unstable," he told ABC TV.
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