An Australian spy agency says it has no way of knowing who has obtained a "highly sensitive" report meant for our top allies after it was allegedly leaked by a junior defence bureaucrat on an online forum.

In an embarrassing security breach, a 21-year-old Department of Defence graduate allegedly managed to download a secret Defence Intelligence Organisation assessment, burn it to a disc, take it home and post it to anonymous image-sharing forum 4chan while praising Julian Assange as his hero.

Defence HQ in Canberra, where the graduate worked. Credit:Andrew Taylor

Court documents describe the discovery of the leak as "fortuitous", occurring only when a former Defence Signals Directorate employee stumbled onto the post while browsing the website.

The report had a marking of "Secret, 5 eyes" on each page, a reference to the intelligence alliance of Australia, the US, Britain, Canada, and New Zealand.