The former president of the Northern Territory Labor Party has been detained at Darwin Airport after returning from the Middle East.
It's believed Matthew Gardiner left Australia this year to join Kurdish forces fighting Isis (Islamic State).
He was stopped by Customs officials at Darwin Airport early yesterday after flying from the Middle East through Sweden and Singapore, the ABC reports.
The Australian Federal Police say they have spoken to a Darwin man on his return to Australia.
"Inquiries relating to his activities while overseas are ongoing and as such it is not appropriate to comment further at this time," the AFP said.
It is not known if he is still in custody.
Gardiner, 43, served as an Australian Army combat engineer in Somalia in the early 1990s, the ABC reports.
Attorney-General George Brandis' office has previously said citizens fighting illegally in overseas conflicts could face up to life in prison on their return to Australia.
"It is illegal to fight in Syria for either side of the conflict," a spokesman said.
"If you fight illegally in overseas conflicts, you face up to life in prison upon your return to Australia.
"We know there are some Australians who think they've made the right choice in becoming involved in overseas conflicts, but that choice only adds to the suffering in Syria and Iraq and it's putting those Australians and others in mortal danger."
Although the Australian Defence Force and Kurdish rebels may share a common enemy, the Australian Government does not recognise the Kurds as a legitimate armed force in Iraq.
Gardiner was stood down as president of the NT Labor Party and had his membership suspended after leaving.
- AAP