Northern Territory Labor Party leader Matthew Gardiner has left Australia to reportedly join the fight against the Islamic State militants in the Middle East.

The Australian Federal Police has confirmed it is investigating the case after the former NT union official and senior NT Labor party figure, left the country.

It is understood the 43-year-old, who recently resigned as NT United Voice union secretary, was heading for Iraq or Syria where he plans to join Kurdish militants.

Gardiner, who has an army background, was allowed to leave the country because he was not on any watch list, ABC reports.

He left behind his wife Andrea and three young children, aged between 3 and 11, who were reportedly completely unaware of his plans before he disappeared.

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NT Labour Party leader Matthew Gardiner has left Australia to reportedly join the fight against the Islamic State

Gardiner (far right) left behind his wife Andrea (far left) and three small children when he fled Australia

He left behind his wife Andrea and three young children, aged between 3 and 11, who were reportedly completely unaware of his plans before he disappeared

Mr Gardiner is also head of the NT Labor Party, though he was suspended from his role after his disappearance.

In February Australian man Ashley Johnston became the first Westerner to be killed fighting against IS for the Kurdish People's Protection Unit (YPG).

It's believed the 28-year-old was fighting with the Kurdish People's Protection Unit (YPG) and trying to liberate the ISIS-held town of Tal Hamis in Syria.

Johnston was reportedly killed after he became outnumbered when a truck he was travelling in broke down in a small village near Shingal, close to Iraq's border with Syrian Kurdistan.

Ashley Johnston, originally from Maryborough in Queensland but had most recently been living in Canberra, told his mother Amanda he was simply heading to the Middle East to do humanitarian work

Mr Johnston's coffin was draped in the Australian and Kurdistan Democratic Union (PYD) party flags, and was carried by men and women dressed in khaki fatigues and scarves in PYD colours of red, yellow and green

Amanda Jonston (centre) the mother of former soldier Ashley Johnston, walks in front of her son's coffin as it is carried out of the chapel during his funeral service at Macquarie Park Cemetery on Thursday

A federal Labor spokesman said opposition leader Bill Shorten was aware of reports regarding Mr Gardiner, but no statement would be made until they were confirmed.

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

Attorney General George Brandis has received an AFP brief on the Gardiner case.

A spokesperson for Mr Brandis said Australians who join militant causes overseas face imprisonment should they survive and return.

Around 90 Australians are understood to have travelled to the Middle East to join militant causes

Gardiner reportedly headed to Iraq or Syria where he plans to join Kurdish militants and fight against IS

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

'It is illegal to fight in Syria for either side of the conflict.

'It is illegal to fight for a terrorist organisation anywhere, including in Iraq.'

Australians who wanted to oppose the actions of the Islamic State group should instead donate to humanitarian groups operating in affected areas, the spokesperson said.

Around 90 Australians are understood to have travelled to the Middle East to join militant causes.

The Australian Federal Police has confirmed it is investigating the case of Gardiner who left the country