If the doorway is set IN common property (the wall) and is leading to common property (the balcony) then it’s common property … unless the building is pre-1974.
This is what the ‘Who’s Responsible’ guide says:
If the strata plan was registered before 1 July 1974 the balcony wall including the windows and door and their working parts are generally part of the lot and the lot owner’s responsibility for maintenance and repairs (unless there is a notation on the strata plan or the balconies are not shown on the strata plan).
If the plan was registered after 1 July 1974, the balcony wall including the windows and doors and their working parts are generally common property and the owners corporation’s responsibility (unless the strata plan says otherwise). With the exception to fly screens and/or security doors where they were installed by an owner after registration of the strata plan