A spokeswoman for NSW Health said the man has recently been in an area with chickens in Vietnam and has had a previous influenza-like illness.
Health experts believe it's very unlikely the man has bird flu but he's been quarantined for testing at a Sydney hospital.
NSW Health says this has been done just as a precaution.
It's believed he was unconscious for most of the Vietnam Airlines flight.
Under Australian Quarantine laws all airlines are required to report ill passengers to the Australian Quarantine and Inspection Service prior to landing.