Hello Tess
You say this is not in common property. If it’s an issue within the floor of the shower recess then it is common property, and any issues within the external walls of the unit are also common property. The original tiling would be considered part of the wall or floor, so an Owners Corporation responsibility in those cases. There are some specialist firms who do this sort of work with a guaranteed result, we’ve had a couple of units with this problem successfully treated, at owners corporation expense within our strata.