Australian authorities have carried out their largest "counter-terrorism" raids so far, detaining 15 people to stop an alleged plot by supporters of ISIL to carry out "demonstration killings" in public.

Australian authorities have carried out their largest "counter-terrorism" raids so far, detaining 15 people to stop an alleged plot by supporters of ISIL to carry out "demonstration killings" in public.

A pre-dawn operation was carried out on Thursday across Sydney and Brisbane by more than 800 officers acting on about 25 search warrants.

The raids came in response to intelligence that an ISIL leader in the Middle East was calling on Australian supporters to kill, Prime Minister Tony Abbott said.

Abbott was asked about reports that the detainees were planning to behead a random person in Sydney.

"That's the intelligence we received," he told reporters. "The exhortations - quite direct exhortations - were coming from an Australian who is apparently quite senior in ISIL to networks of support back in Australia to conduct demonstration killings here in this country."

Police were holding six people and have identified the suspected ringleader, officials said. Nine other people were detained but were freed before the day was over.