Felix – common walls, floors, and any original covering on them such as waterproofing, tiles, magnesite, but not paint are the responsibility of the Owners Corporation (O/C), and so conversely any internal walls and their coverings are the responsibility of the Owner.
A general interpretation is that a wall of the shower and anything within it such as plumbing that’s also a common wall such as one on the perimeter of the building or adjoining another lot is the responsibility of the O/C, and a wall of the shower and anything in it that’s internal such as one separating it from a bedroom or a kitchen of the lot is the responsibility of the Owner.
There’s a more detailed guideline HERE to assist in interpreting who’s responsible for what in Strata.
There’s an issue here that I in part canvassed in THIS earlier post, where there’s an argument that Owners should be held responsible for the entirety of repairs where a lack of maintenance and/or timely advice of a problem to the O/C or its Strata Manager made the extent of damage and therefore the cost of repairs higher than it would otherwise have been (i.e. with more timely advice).
If your O/C decides upon the stance that I took with our Plan, again as outlined in my earlier post, then it’s best to leave subsequent discussions about who didn’t inform who to the Owner and their Property Manager (rental agent).