you could argue that based on the fare calculation that the origin is YYZ and the destination is SYD if... your fare calc reads YTO AC X/LHR AC X/YTO AC SYD. Personally since there is no YYZ-SYD flight I assume your fare calc actually reads YTO AC X/LHR AC X/YTO AC X/YVR AC SYD. If so then you have an argument for 1 cert in each direction. Good Luck!