Rydel – As Jimmy hasn’t responded as yet, I’ll offer an opinion.
The previous Proprietor who completed the upstairs renovations should have received the consent of your Owners Corporation (O/C) before undertaking those works, and IF that procedure was followed, it’s usual for the O/C’s consent to include a provision to the effect that all ongoing maintenance and repairs to the items comprising the renovations is the responsibility of the Proprietor of the Lot (i.e including subsequent Proprietor/s).
This procedure is in place to safeguard the (structural) integrity of the Common Property during the renovation process and additionally to protect the O/C from any future expense in circumstances where, as examples, the original “developer-quality” tiles may have been replaced by a Proprietor with high-quality “expensive” tiles, or where the waterproofing has not been completed by a Contractor known to and/or engaged by the O/C.
All the above is dependent upon the Proprietor and your O/C following the correct procedures, so your first inquiry should be of your Strata Manager concerning the existence or otherwise of evidence supporting those procedures.
If as often happens, the consent of the O/C was neither sought nor given, then the repairs to and the replacement of the waterproofing and tiles on Common Property areas such as the floor and non-internal walls within the renovated area are, by default, the responsibility of the O/C.
Repairs and replacements to tiles and waterproofing to other areas of the bathroom are the responsibility of the current Proprietor of the Lot.