Matthew Gardiner, who also served as a secretary in the United Voice union and a treasurer in peak body Unions NT, left the country several weeks ago to fight with Kurdish militants, the ABC reports.

It's believed Mr Gardiner, 43, who served with the Australian Army in Somalia in the 1990s, was able to leave Australia because he was not on any watch list.

It is illegal to fight in Syria on any side of the conflict against Islamic State.

It's believed 90 Australians are fighting in the conflict.

A spokesperson for Attorney-General George Brandis told the ABC foreign fighters face severe penalties regardless of which side they are fighting with.

"If you fight illegally in overseas conflicts, you face up to life in prison upon your return to Australia," the spokesperson said.

"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."

It appears Mr Gardiner made recent connections on social media with people sympathetic to Kurdish militias, who are fighting Islamic State.

Although Mr Gardiner resigned from his position at United Voice, he remains the NT branch president of the Labor Party. He has not been seen for weeks and his mobile phone is switched off.

The matter is being investigated by the Australian Federal Police.