The head of the Northern Territory branch of the Labor Party has reportedly flown overseas to fight in Syria against terrorist group Islamic State.

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.

Matthew Gardiner, a senior figure in the Northern Territory branch of the Labor party, has reportedly gone to fight in Syria. Credit:ABC TV

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.