SYDNEY (AFP) - As Australian troops join United States-led efforts to combat the Islamic State in Iraq and Syria (ISIS), one of the country's political figures has reportedly taken matters into his own hands and gone to the Middle East to fight them himself.

Mr Matthew Gardiner, a former trade unionist and former president of the Northern Territory branch of the opposition Labor Party, left Australia to join a Kurdish militia fighting ISIS, the Australian Broadcasting Corporation reported on Sunday.

The 43-year-old, whose whereabouts are now being investigated by the police, stands in marked contrast to around 90 Australians whom Attorney-General George Brandis says have travelled to Iraq and Syria to fight, principally alongside ISIS militants.

The Australian government issued a warning at the weekend that young people including women were still being drawn to the conflict, despite new laws targeting people who fight overseas.

Labor leader Bill Shorten said on Monday that Mr Gardiner had made a mistake.

"Whatever the guy's motivations, you're not going to solve things by going there," Mr Shorten told Fairfax Radio.

"I think it's come out as a bolt out of the blue. He's served in the military previously, he's been a union organiser, he obviously feels very strongly about fighting ISIL but I don't think that's the right way to go about it, just to up sticks," Mr Shorten said, using another acronym for ISIS.

"I think he's made a mistake."

The ABC said Mr Gardiner served as an Australian Army combat engineer in Somalia in the 1990s.

Mr Gardiner, who is reported to be married with children, was removed as the Northern Territory chief of the Labor Party and suspended from the organisation after the news emerged, the ABC reported.