University of NSW law professor George Williams said on Monday a suspect would likely have to confess for there to be a solid case in so-called "foreign incursions" laws, given the difficulty of obtaining evidence from a war zone.

Mr Gardiner – a former president of the ALP in the Northern Territory - returned home at the weekend after several months abroad during which he was widely reported to have been in Syria fighting with Kurdish forces against the Islamic State terror group.

The Australian Federal Police have confirmed they detained and questioned Mr Gardiner on his return to Australia on Saturday morning but released him without charge. An AFP spokesman said on Monday the force had no further comment at this stage.

A spokeswoman for Attorney-General George Brandis likewise declined to comment, saying it was a law enforcement matter.

Professor Williams said prosecutions for fighting with groups abroad would always be difficult – including people fighting for the Islamic State and other extremist groups.

"It just shows that a law can be impotent," Professor Williams said. "It depends on getting evidence, and getting evidence from war-torn areas is difficult at the best of times, let alone in Syria or Iraq right now."

If people "remain silent and do not offer any admissions", then "it's an almost impossible task for the prosecutors", he said.

Top barrister and former national security watchdog Bret Walker, SC, similarly said prosecutors faced an uphill battle in launching such cases.

"They're going to have to show that he was doing, and with the appropriate intention … things which fall within the prohibited category. And that's not very straightforward unless you've got some reliable eyewitnesses, and hearsay won't do," Mr Walker told ABC radio.

He said that when the AFP interviewed Mr Gardiner, "they were probably seeing whether he would make any admissions".

It has long been a crime for Australians to engage in hostilities overseas with a group that is not the legitimate armed forces of a foreign country. But the government increased the penalty for the offence to life imprisonment in its recent crackdown on so-called "foreign fighters". It also introduced a new offence for merely being in terrorism hotspots and has already declared al-Raqqa province in Syria and the Mosul area in Iraq as no-go zones.

However, the intention was to prosecute the hundreds of Australians fighting with Islamic State and other jihadist groups, not isolated cases where citizens are fighting with putative allies of Australia such as the Kurds.

Professor Williams said the current law did not sufficiently discriminate between who a person fought for, or how serious their involvement in hostilities was.

"It applies life in prison to a very wide variety of circumstances, including merely preparing to do this - you need not have even left Australia. It would be better if the offence were narrower with graduated penalties."

He said the government should focus on counter-radicalisation programs to stop Australians leaving to fight in the first place.

Calls to Mr Gardiner's home in Darwin and his wife's mobile phone went unanswered on Monday.