A teenager who admitted to planning a terrorist attack in Sydney in 2024 has told the court he was radicalised online after first seeing extremist material on TikTok and used ChatGPT to form his plans.

The 16-year-old, known under the pseudonym as RK due to his age, pleaded guilty to acts in preparation for, or planning, a terrorist act. In a sentencing ruling on Monday, he avoided jail time, with the New South Wales supreme court ordering him into a three-year community corrections order.

RK had just turned 14 when the offending occurred in 2024. He told the court he was first exposed to extremist material on TikTok.

“Initially, he was looking at historical content that may have had a political theme. He saw videos about the crusaders and ‘their empires’,” Justice Stephen Campbell said in the decision.

“He progressed to videos about the events of September 11, 2001 and the resulting war on terror (and the wars after that).”

Campbell said RK then progressed to watching religious-themed videos, which “led him to other content of increasing violence” and then talking about these subjects with people on the internet.

RK told the court he was trying to impress these people, and became focused on the Sydney Opera House as a potential location for an act of violence. He said he had become radicalised and “brainwashed by propaganda” when he began looking at bomb-making videos on YouTube, but Campbell said he had no concrete plans for committing a terrorist act, and had not obtained any material for bomb-making.

RK used Telegram, seeking instructions on making a pressure cooker bomb, and attempted to contact the surviving Boston marathon bomber, Dzhokhar Tsarnaev, by email.

The teenager used ChatGPT to obtain information about the bomber, and asked the AI chatbot if inmates had access to news.

“Exhibiting what appears to me to be a complete lack of understanding of the import of his conduct, the young person emailed the prison, using a publicly available email address, asking whether he could email Tsarnaev,” Campbell said in his judgment, adding RK never ultimately made contact with the bomber.

RK asked ChatGPT about security arrangements at the Sydney Opera House, where the Sydney marathon finishes, and where terrorists were imprisoned in NSW. He watched videos about the Sydney Opera House on YouTube, and researched gay nightclubs and other places where “infidels” gather.

On Telegram, RK sent Islamic State videos to others to “seek validation” because he thought it was “cool”, Campbell said.

OpenAI, Telegram and YouTube were approached for comment. TikTok declined to comment.

The teenager has since expressed remorse for his actions, and Campbell said he had underwent six sessions of religious and ethical training from a Muslim cleric.

“I am satisfied that he has genuinely renounced his adherence to Islamic violent extremism, which in any event was incipient and hesitant during the short period of his offending,” Campbell said. “I fully accept he was radicalised to a degree, but that the process of deradicalisation is well advanced.”

The Asio deputy director general for intelligence service delivery, Claire McGuinness, told a parliamentary inquiry last month that detecting and disrupting online youth radicalisation had become very challenging for the security agency.

“It usually begins quietly. A young person is scrolling through Instagram or TikTok or a similar platform,” she said. “They encounter something that catches their attention, perhaps a political meme or a religiously conservative video.

“At first it isn’t terrorism – it’s just fringe; it is extremist-adjacent. But, the moment they engage, the algorithm takes over and so the feed tightens, the content shifts from conservative to radical, and from radical to security-relevant.”

From there, teenagers often move to platforms such as Discord or Telegram where “dark humour and the edgy memes” groom teens to accept progressively more extreme content, McGuinness said, before passive consumption turns to active engagement in researching methods.