A plan by Islamic State to conduct public executions in Australia sparked police raids, says Prime Minister Tony Abbott.

A series of anti-terrorism raids were sparked by intelligence reports the Islamic State was planning a public execution in Australia, Prime Minister Tony Abbott says.

Federal and state police executed warrants in Sydney and Brisbane on Thursday which resulted in 15 people being detained and one man charged in Sydney with terrorism-related offences.

Mr Abbott was briefed on the police raid on Wednesday night, which included intelligence that public beheadings were planned.

“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,” he told reporters.

“So this is not just suspicion, this is intent and that’s why the police and security agencies decided to act in the way they have.”

Mr Abbott said Australia was at “serious risk” of a terrorist attack and Australians were supporting extremists who would do the country harm.

“It’s important our police and security organisations be one step ahead of them and this morning they were,” he said.

RAAF aircraft and commandos are heading to the Middle East before being deployed to an international mission in Iraq.

Mr Abbott will farewell RAAF members at Williamtown and Amberley bases on Thursday afternoon before heading to Sydney for national security briefings.

He said he would like to return to Arnhem Land, where he has been based all week.

“But at this point it would be wrong of me to make a specific commitment,” Mr Abbott said.