Matthew Gardiner was also the secretary of hospitality, childcare and emergency services union United Voice in the NT, as well as the treasurer of peak body Unions NT.

The 43-year-old served as Australian Army combat engineer in Somalia in the early 1990s.

The ABC understands Mr Gardiner was able to leave the country because he was not on any watch list.

About 90 Australians are now thought to be overseas fighting with Islamic State or similar terrorist groups, which has prompted the Government to repeat its warning against people joining the conflict.

A spokesperson for Attorney-General George Brandis 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 spokesperson urged people concerned about the conflict to donate to charities.

The Australian Federal Police has refused to discuss any detail, saying the matter is part of an ongoing investigation.Mr Gardiner recently befriended a woman on Facebook whose page is littered with support for Kurdish militias fighting Islamic State.

A spokesperson from United Voice's national branch confirmed Mr Gardiner had resigned, but the ABC has confirmed he is still the president of the NT branch of the Labor Party.

Colleagues contacted by the ABC had been suspicious, with Mr Gardiner not being seen for weeks and his mobile phone has been switched off.

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