Problems with defects in the waterproofing at the construction stage are very common.
https://www.choice.com.au/money/property/buying/articles/apartment-building-defects
The statutory warranty period for the 22 year old building has expired long ago.
From my reading of the issue of waterproofing it was only from 2004, that the Australian Standards required waterproofing membranes to be applied to shower stalls and across bathrooms. Check with a plumber though.
Also, the new codes require that baths with an integral vertical upstand lip along the side of the bath walls are required to be recessed to enable the junction to be waterproofed. Baths without an integral edge require
full waterproofing of the walls and floor area around and under the bath.
I am not sure whether there are stricter rules for apartment buildings than there are for normal residential houses.
WR means water resistant. WP means waterproof.
