Before attempting to identify whether any item in a strata scheme is common property or part of a lot you must look at:
The registered strata plan, specifically the floor plan, is the only document that legally defines the boundaries of the lots in the scheme. Every thing that is not part of a lot is common property. Each strata plan can be different. However, in a typical strata plan the following principles generally apply:
In view of the above principles, tiles on a bathroom wall that is shown by thick linework on the floor plan or tiles on a bathroom floor, are common property provided that they were in place at the date of registration of the plan. It is possible for some bathroom walls (including the tiles) to be common property and others to be part of the lot.
A current search of the Certificate of Title for the Common Property should be always be obtained to ensure that there has been no change to the strata scheme, which will effect the above determinations. Registration of action such as strata plans of subdivision and changes of by laws may affect common property or issues of maintenance and repair of common property.
The above general principles are not restricted to bathrooms and apply to tiles affixed to any surface in the strata scheme.