Two males picked up in counter terrorism raids in Sydney had travelled or tried to travel to Syria, police say.
They include 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 Islamic state.
The NSW Joint Counter Terrorism Team carried out raids in Sydney's southwest early on Thursday.
"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, Ms 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.
Mehmet Biber in police custody Source: NSW Police
The 17-year-old was stopped from travelling to the airport by police on Thursday, said Mr Gaughan.
It's believed the older male left Australia to join the terrorist group Jabhat al-Nusra, or Nusra Front, in Syria in 2013 before returning in 2014.
It's also believed he was recruited to fight in Syria by Hamdi Alqudsi, who was sentenced two months ago in the NSW Supreme Court to at least six years behind bars for arranging for seven men to make their way to the battlefields.
"If anybody returns to Australia ... who has engaged in hostile activities and we are aware of them, we will continue to either monitor or do something with them and if we haven't got the evidence to put before the court, we will use other strategies to make sure our community is safe," Ms Burn said.
Mr Gaughan believes around 110 Australians are suspected of being fighting in the region.
"There is a large number that have been killed, close to 60.
"We still see people attempting to leave, but the numbers have slowed down significantly.
"It is very unfortunate that even though ISIS is losing significant ground over there that we still have young people, particularly young people, that are attracted to the ideology and are still attempting to leave the country."
Both males are expected to face Sydney courts later on Thursday.
