The Owners Corporation is responsible for the repair as the ducting encloses structural cubic space. Structural cubic space is automatically common property even though it may exist within the cubic space of your lot, unless the strata plan specifically states otherwise.
Structural cubic space means
- cubic space occupied by a vertical structural member, not being wall, of a building (e.g column, post, pole, etc);
- any pipes, wires, cables or ducts that are not for the exclusive enjoyment of one lot; and
- any cubic space enclosed by a structural enclosing any such pipes, wires, cables or ducts.
On this basis the Owners Corporation is responsible for repair & maintenance of the timber ductwork and the sewer pipe.