COULD there be any better fodder for Australia's conspiracy theorists? The Department of Defence has ''lost'' its X-Files.

For decades, Defence officials dutifully investigated an unknown number of UFO sightings being reported all over the country. Intelligence officers attached to the Royal Australian Air Force checked the known movement of aircraft against reported sightings, and politely responded by mail to everyone who claimed they saw floating lights or flat saucers or streaking vapour trails. Some of the files were marked as classified.

Last year, the British government released a large part of its dossier on unidentified flying objects after significant pressure from freedom- of-information applicants. More than 4000 pages of Defence Ministry documents, detailing 800 reported encounters during the 1980s and 1990s were posted online.

Fairfax sought access to the Australian version. But the response was more surprising than what the files might have contained - the material has largely gone missing.

The department spent two months searching its offices for files that would be captured by the Herald's FOI application, which sought a ''schedule of records held by the Department of Defence … which relate to unidentified flying objects''.