nellv – before your post drops off the bottom of the list, I’ll offer some advice.
It’s the Owners Corporation and not you who engaged and is presumably paying for the builder’s services, so it’s up to it to resolve any consequent issues such as the one with the waterproofing of your bathroom.
What you need to do is to get yourself out of the middle, and to insist in writing that your Owners Corporation (O/C), via its Executive Committee and its Strata Manager, deals with the builder and their minions to have your problems resolved within a timeframe that’s acceptable to you.
If you’ve put that in a formal way to your E/C Secretary and Strata Manager already and its documented, then you can immediately apply to have the matter Mediated, and if that doesn’t achieve an acceptable outcome then you can apply for Orders to force the O/C to exercise a function for which its responsible (and for which its accepted responsibility by virtue of its special levy) on the grounds that it hasn’t done so within 2 months of your request regarding the repairs to your Lot; see Sect 138(2)(b) of the NSW Strata Schemes Management Act (1996).
Finally, with regard to your question about the O/C absolving itself of any responsibility to maintain an area of the common property in your Lot (i.e. within your bathroom), YES it can resolve to do that by way of a 75% majority vote at a General Meeting, and particularly so if you engage another builder to attend to your long-awaited repairs; so however frustrated and annoyed you become DON’T DO THAT!