Australia’s spy chief Duncan Lewis says up to 49 Australians may have been killed in Syria and Iraq and the age of some homegrown terrorists and supporters is “astonishingly young”.

ASIO director-general Mr Lewis told Senate estimates that 45 people had been confirmed dead but the number could be higher.

Mr Lewis said almost 200 Australians were actively supporting the terrorist group Islamic State at home, and 110 Australians were overseas fighting in the conflict.

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“The demographic is young. If I was talking to you a couple of years ago typically we would have been talking about people in their late 20s, early 30s,” he said.

“By the start or middle of last year we were … down to the teens.

“If you asked me for a median without being precise I would say it’s in the early 20s, but the trend is down and we do have at the bottom end of that spectrum some people of astonishingly young age.

“Untrained and naive young Australians are being drawn into the conflict and finding themselves in what I would describe as highly expendable, highly dangerous positions of low importance amid the [IS] effort,” he said.

Mr Lewis said along with three successful terrorist incidents on home soil, ASIO had thwarted six planned attacks, adding the plans were low-tech and unsophisticated.

The committee also heard the overwhelming majority of those supporting or financing terrorism were Australian-born.

ASIO said it had provided adverse security assessments that resulted in the cancellation or refusal of 156 passports.

-ABC