Under the terms of The Strata Schemes Freehold Development Act 1973, any wall (including the door and door locks) between a unit and its balcony is part of the relevant lot if the building was registered prior to the 1st of July 1974. Ask your Strata Manager to check the registered strata plan for this building and confirm if the building was registered prior to 1.7.1974. If it was, this means that the maintenance of the balcony door (including replacing the locks) would fall under the responsibility of the individual owner, not the Owners Corporation, unless there is a specific notation on the Strata Plan. If the Strata Plan was registered after 01.07.1974 then submit the invoice to the Strata Manager for payment. We recommend that you consult your Strata Manager to confirm the date of registration prior to contacting any contractor to attend to the works.