Carlo Konstantin Kohl, 25, was being extradited to Germany via Thailand when bad weather forced an extended stopover at Bangkok's Suvarnabhumi airport and his two Australian guards decided to take him to a transit lounge.
During the overnight layover Kohl, convicted in Australia of stealing opals, drug trafficking and wanted for skipping parole in his native Germany, managed to give his escorts the slip and escape the airport.
Immigration department chief Martin Bowles confirmed the May 15 incident but denied Thai media reports that the two guards, from a private firm that runs Australia's immigration security, were asleep when he got away.
"Contrary to the media report the escorts were not asleep -- (that's) what I've been told," Bowles told a parliamentary hearing.
Conservative Senator Eric Abetz blasted Bowles for the "pretty big and embarrassing issue for Australia as a nation, let alone for the department administering this".
According to Thailand's Bangkok Post newspaper, Thai police were not informed that Australia was transiting Kohl through Suvarnabhumi.
"All the bureau can say now is that suspect is not in the airport," Suwichpol Imjairach, division two chief of the immigration bureau, was quoted as saying.
"But it remains a mystery where he is now and how he managed to get out of the airport."
The Sydney Morning Herald said a Thai police probe had concluded the guards fell asleep and Kohl fled the airport by slipping through a fire exit door he disarmed by disconnecting the electricity.