The arrests took place during several raids by the state and federal anti-terrorist police units in different areas of Sydney in New South Wales, Efe news reported.
Assistant Commissioner Neil Gaughan said that Mehmet Biber, 24, was accused of joining the Al-Nusra Front, affiliated to Al Qaeda, in July 2013 before returning to Australia six months later.
The other detainee is a 17-year-old minor who tried to travel to the Middle East to join the Islamic State in 2015, said Gaughan.
“Obviously we’ve moved as quickly as we can in relation to this matter due to the risk that these people pose,” Gaughan added.
Authorities estimate around 110 Australians have traveled to the Middle East to join jihadi fighters including 60 who have died in combat.
Australia raised the terrorism alert to high on September 2014 after recording a rise in young people being manipulated by Islamic extremism engaged in terrorist attacks on Australian soil or attempting to leave the country for combat in the Middle East.