Two men were charged in Australia on Thursday over breaches of the country's foreign fighter laws, with one accused of joining the Al-Nusra Front in Syria.

It followed a series of morning raids across Sydney by the New South Wales Joint Counter Terrorism team, with the other person detained after allegedly trying to travel to Syria to fight with the Islamic State group.

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."

Mehmet Biber, 24, was charged with "incursions into foreign states with the intention of engaging in hostile activities" and if convicted faces up to 20 years in jail.

Police claim he left for Syria in July 2013 to join Al-Nusra -- the former name of ex-Al-Qaeda affiliate Fateh al-Sham Front -- before returning to Australia six months later.

A 17-year-old boy, who cannot be named because he's a juvenile, was charged with attempting to travel to a conflict zone and encouraging others to do the same, for which he could face a maximum penalty of life in prison.

They were both refused bail and are due to reappear in court on December 15.

Their arrests follow the recent sentencing in Australia of Hamdi Alqudsi, who was convicted of recruiting seven men and facilitating their entry into Syria to fight for Al-Nusra. He was jailed for six years.

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

"We still see people attempting to leave, but the numbers have slowed down significantly," he said.

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.