The age of Australians being radicalised by the Islamic State (IS) group is increasingly getting younger, the country's top spy has warned.
Data showed a significant rise in young suspected extremists from 2013 to 2015, said Australian Security Intelligence Organisation (ASIO) chief Duncan Lewis.
He said adherents of an extremist interpretation of Sunni Islam posed the greatest terror threat to Australia.
But he stressed "99.9% of Australian Muslims" were of no interest to ASIO.
Mr Lewis said in 2013, 45% of suspected Sunni Islamic extremists were aged between 25 and 34. Two years later, 40% were aged between 15 and 24.
"It basically dropped by a decade in the space of a couple of years," he told a senate estimates hearing on Tuesday night.
"We are still looking at a very young cohort that are impacted and influenced by this... extremist, violent message."
He said the trend would continue to affect Australia's security environment.
"The other 99.9% of Australian Muslims are not involved in activities of security concern in any way and are of no interest to the Australian Security Intelligence Organisation," Mr Lewis said.
His comments came hours after a 42-year-old Australian man was arrested on suspicion of trying to advise IS on missiles.