The former president of the Northern Territory Labor party has been released after he was detained at Darwin airport after returning from the Middle East, reports the ABC.
It's believed Matthew Gardiner left Australia earlier this year to join Kurdish forces fighting Islamic State.
He was stopped by Customs officials at Darwin airport early on Sunday, 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," an AFP spokesperson said.
It is not known if he is still in custody.
Mr Gardiner, 43, previously served as an Australian Army combat engineer in Somalia in the early 1990s, the ABC reports.
In a statement a spokesperson for Attorney-General George Brandis made it clear that even though Mr Gardiner was fighting against IS he would be in breach of Australian law.
"It is illegal to fight in Syria for either side of the conflict," the spokesperson 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."
Federal Labor leader Bill Shorten told the ABC he was relieved Mr Gardiner was "back home safe and sound".
"But I'm concerned anyone thinks they should be getting involved in these foreign conflicts, no matter what their intentions," he said.
