Suspicion on Gardiner's whereabouts were roused when he had not been seen for several weeks with his mobile phone switched off. The Australian Federal Police have refused to comment on the issue, saying an investigation was ongoing, according to local media.
Meanwhile, a spokesperson for Australian Attorney-General George Brandis said "it is illegal to fight in Syria for either side of the conflict," as quoted by ABC.
The spokesperson added that those travelling to fight in overseas conflicts could face a life sentence up on return.
According to the news outlet, about 90 Australian nationals are believed to be currently fighting within the ranks of the Islamic State and other terrorist groups.
The Kurdish fighters have been battling IS at the Syria-Turkey border town of Kobani since fall 2014, aided by the ongoing US-led coalition's airstrikes. Recently, the Kurdish fighters also began firing rockets into the IS-controlled city of Mosul in Iraq.
The extremist militant group began fighting the Syrian government in 2012, later expanding its operations to Iraq, and declaring a caliphate over territories it controlled in both countries in 2014. The IS is infamous for its brutal tactics, such as public crucifixions and beheadings of captured prisoners.
