AFP 'should probe Gardiner's travel': Shorten

Opposition Leader Bill Shorten says former Northern Territory Labor party president Matthew Gardiner, who is believed to have travelled to the Middle East to support Kurdish forces fighting Islamic State, should be investigated.

The Australian Federal Police today said investigations were continuing into Mr Gardiner's activities, after questioning the 43-year-old for several hours on Sunday following his arrival at Darwin airport.

Mr Gardiner, who served as an Australian Army combat engineer in Somalia in the early 1990s, was stopped at customs.

He had travelled from the Middle East, via Sweden and then Singapore, before arriving in Darwin.

It's understood Mr Gardiner, who has a wife and two sons, travelled to the Middle East and possibly to Syria to support Kurds fighting Islamic State.

What role he might have played, or if he may have breached Australian foreign-fighter laws, is unclear.

Syria is also a "declared area", which means Australians are prohibited from travelling there without a legitimate purpose.

Mr Shorten said Australians who travelled to conflict zones in Syria and Iraq should be appropriately investigated.

"We say to any Australian who thinks they should take part in these conflict zones, don't do it. And if you return the police will appropriately investigate the matter consistent with the law," he said on Tuesday.

"It is illegal for Australians to support armed groups in northern Iraq and Syria."

Attorney-General George Brandis has previously warned Australians fighting illegally overseas could be be jailed for life.

Mr Gardiner, according to the ABC, says he had been given strong legal advice to say nothing about the case.

He also reportedly has concerns for the safety of his family, and has told the ABC he fears revenge attacks.

The AFP has said it will not comment further on the case while its investigation is ongoing.