Young people who become radicalised quickly flip between ideologies, and in that case the teenager displayed "an obsession with violence" rather than adhering to a particular cause, Home Affairs Department deputy secretary Nathan Smyth said on Wednesday.
"It's ideological promiscuity," Mr Smyth told an Australian Institute of Criminology conference in Canberra.
"Some young people are attaching themselves to an ideology, or mix of ideology, or no ideologies, but they're motivated by personal grievance.
"The lack of protective factors out there are really kind of accelerated and amplified by social media."
Young people today spend the majority of their lives online, including during their most formative years.
Some are entering age groups that make them vulnerable to radicalisation online, Mr Smyth said.
They are being targeted on social media, and particularly on gaming platforms.
Mr Smyth pointed to the live-streamed video of the non-fatal stabbing of a bishop in western Sydney in 2024.
It was later cited as a factor in an attack that ended in the murder of three children in the UK town of Southport months later.
The perpetrator had viewed the Sydney stabbing video hours before the killings.
"Southport is a very clear example of the real-world influence of online terrorist and violent extremist content," Mr Smyth said.
"It shows how people can be radicalised to violence through extremist views that have formed online and happen very, very quickly."
The Sydney stabbing was also used to amplify misinformation and disinformation online.
"We're already seeing AI-fuel disinformation and deepfakes that are promoting false narratives, as well as terrorist and violent extremist narratives," Mr Smyth said of the online environment more broadly.
"It undermines factual information, it ferments social division and erodes trust in our institutions."
Foreign powers were known to manipulate social media and online platforms to "artificially promote their popularity, the reach and the volume of the content that supports their narratives", the counter-terrorism co-ordinator told the conference.
"Narratives promoted by foreign interference don't need to become dominant," he said.
"Their continued presence out there with social media degrades democratic discourse, it creates and exacerbates social divisions and it impacts our national security."
Young people, especially those aged between 18 to 34, are displaying more conspiratorial views and this wasn't subsiding despite the end of the pandemic, the Australian Institute of Criminology's Tim Cubitt told the conference.
While the overall percentage of people on social media viewing radical content online dropped significantly between 2022 and 2024, those who intentionally accessed such content said it depicted violence.
"Once they accessed it, they were more likely to go back and access it," Dr Cubitt said.
Those who accessed radical content were more likely to agree it was acceptable to use violence in support of a collective goal, with almost a quarter of 18 to 34-year-olds agreeing, he added.
Those who accessed radical content were also more likely to report being dissatisfied with democracy, he said.
