'Fighter against IS' released in Darwin

The former president of the Northern Territory Labor party, who is believed to have been overseas fighting Islamic State, has been released without charge after being detained upon return to Australia.

Matthew Gardiner reportedly left Australia earlier this year to join Kurdish forces and was stopped by authorities when he arrived at Darwin airport early on Sunday.

He was later released without charge, an AFP spokesman said.

"Enquiries relating to his activities while overseas are ongoing and as such it is not appropriate to comment further at this time," he said.

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

Australians fighting illegally overseas can be jailed for life, Attorney-General George Brandis's office has previously warned.

Senator Brandis declined to comment on Sunday, with a spokesman saying it was inappropriate as the matter was under investigation.

Federal Labor leader Bill Shorten told the ABC he was relieved Mr Gardiner was home but concerned he had been fighting overseas.