The file claimed to be the preview edition of Leopard given to attendees at Apple's annual Worldwide Developers Conference, held in San Francisco last week.
But participants at the conference are prohibited from disclosing some information presented during the proceedings by non-disclosure agreements.
The file found on several popular BitTorrent file-sharing Web sites was labelled "APPLE MAC OSX LEOPARD V10 5 WWDC PREVIEW-BETAOSX 443061 499".
ZDNet Australia has as of yet been unable to download the file to verify if it was legitimate or simply a hoax.
Macintosh users are hotly anticipating the release of Leopard, which will include a slew of new features and is due to make it to the market early next year.
Apple chief executive Steve Jobs gave the public its first glimpse of Leopard during his keynote at the WWDC last week, simultaneously announcing several other products, including server and desktop hardware based on Intel's new dual-core CPUs.