Australian police on Thursday arrested a man believed to have fought with Syrian militants and a youth who was stopped at the border allegedly planning to join Daesh terrorist group in Syria.

It followed a series of raids across Sydney by the New South Wales Joint Counter Terrorism team underscoring Australia's stance on dealing with increasing foreign fighters heading overseas to join in conflicts in countries including Syria and Iraq.

Australian Federal Police assistant commissioner for counter terrorism Neil Gaughan said it had been a protracted investigation as gathering evidence from Syria was "extremely difficult".

"We have arrested these gentlemen as quickly as we possibly could," he said, adding that those returning from either Syria or Iraq could carry out violent acts once back in Australia.

"The challenge for our members is to gather evidence to an appropriate standard to enable a prosecution," he said.

The 24-year-old man and the 17-year-old youth were both charged with breaching Australia's foreign incursion laws, which prohibit travel to foreign countries with the intention of engaging in hostile activity. Those found guilty face up to 25 years imprisonment.

Gaughan said 110 Australians were believed to have travelled to Syria or Iraq to join terror groups, of which around 60 have been killed.

Australia has imposed some of the harshest penalties in the world on foreign fighters returning home as the country leads a push for the United Nations to adopt international standards to curb the threat they pose.

Australian officials say they have prevented 11 terror attacks on home soil in the past two years. But several have taken place, including the murder of a Sydney police employee last year.