In a speech marking a quarter of a century since nearly 3000 people were killed in the largest terror attacks on American soil, ASIO director general Mike Burgess said while the nature of terrorism had changed, some elements have come back.
"Twenty-five years later, transnational terrorism has re-emerged as a significant, malignant security concern," Mr Burgess said in a speech on Friday.
"Al-Qaeda, ISIS and their affiliates are exploiting permissive spaces in the real world and online to grow their capabilities, demonstrating a renewed capacity to conduct and inspire attacks outside their immediate spheres of influence.
"These groups still see western interests as legitimate targets."
Mr Burgess said ASIO had expanded significantly since the attacks on the World Trade Center and the Pentagon in 2001.
The director general revealed ASIO had foiled 33 major terrorism plots since 2014 where an attack was imminent or planning for one was in advanced stages.
Of those, 16 have been disrupted since the Bondi mass shooting in December 2025.
"The alleged cases include a planned anti-Semitic attack, threats to kill Muslims, and an advocate of racist violence allegedly found in possession of weapons and ammunition," Mr Burgess said.
"Of course, ASIO is not all seeing and all knowing. And we do not want to be. We cannot stop every terrorist, just as we cannot catch every spy. But we will continue to work around the clock to keep Australians safe."
The ASIO head said while Sunni violent extremism represented the majority of the case load of the spy agency's work, the terror threats had diversified in the 25 years since September 11.
"It is too simplistic to assume there is a single terrorism threat or a most likely terrorist threat as we did in the aftermath of 9/11," he said.
"Sophisticated mass-casualty attacks have largely been supplanted by individuals or small groups using basic weapons and tactics, making it harder for authorities to detect and intervene."
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