Former president of NT Labor, Matthew Gardiner, has been released by Darwin authorities without charge after returning from helping Kurdish fighters against Islamic State militants in the Middle East.

Mr Gardiner, who left Australia earlier this year, was stopped by Customs officials at Darwin airport early this morning after flying from the Middle East via Sweden and Singapore.

The Australian Federal Police (AFP) confirmed they spoke to a Darwin man who returned to Australia on Sunday morning.

An AFP spokesperson said enquiries relating to the man's activities while overseas were ongoing and they would be providing no further comment.

Gardiner, 43, previously served as an Australian Army combat engineer in Somalia in the early 1990s.

He was stood down as president of the party and had his membership suspended after leaving, according to a statement from Territory Labor.

A spokesperson for Attorney-General George Brandis said it was not appropriate to comment on Mr Gardiner's return as it was now a law enforcement matter.

Senator Brandis has previously said it was illegal for Australian citizens to support any armed group in Syria, even though the Australian Defence Force and Kurdish rebels may share a common enemy.

"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."

The Australian Government does not recognise the Kurds as a legitimate armed force in Iraq, the spokesperson said.

Federal Opposition Leader Bill Shorten said 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.

"The message has to be to Australians: We're not going to fix those issues by becoming a foreign fighter and the law's going to have to take its process."