Hi Phelbe, unless your Plan has a By-law to the contrary, repairs to common wall/s within the bathroom (both sides in your case) and to the bathroom floor only are the responsibility of the Owners Corporation (O/C).
A word of caution though – whilst it’s appropriate for the O/C to coordinate the repairs, all areas repaired and paid for by the O/C become its on-going responsibility. So ensure that the Contractor/s who quote for and undertake those repairs provide two (2) price schedules; one for those works for which the O/C is responsible and the other for all remaining works for which the Lot Owner is responsible, with the resultant invoices being then paid separately by the parties.
In that way the status quo is preserved, in that the O/C’s on-going responsibilities are as they were prior to the waterproofing works.