Two males picked up in counter-terrorism raids in Sydney had travelled or tried to travel to Syria, police say.
They are a man, 24, and a boy, 17, and both have been charged with foreign incursion offences.
The man is linked to al-Nusra, an affiliate of al-Qaeda, while the teen tried to join Isis (Islamic State).
"They will face the full force of the law," Australian Federal Police Assistant Commissioner Neil Gaughan said.
According to media reports the man who has previously fought in Syria was arrested in Birrong after his car was intercepted by police.
The NSW Joint Counter Terrorism Team carried out raids in Sydney's southwest.
"Searches are still under way at Bass Hill, Merrylands, Swansea Park and Miller and at this stage we don't know the outcomes of those searches," Deputy Commissioner Catherine Burn told reporters.
The arrested teenager wanted to engage in hostile activities overseas but was stopped from leaving the country, Burn said.
People who travelled to either Syria or Iraq - or some other place where there are hostile activities - are more susceptible to carrying out violent acts back home, she said.