I was a bit confused between balconies and terraces, too.
FYI: You can’t assume that balconies are the responsibility of the lot owners – in fact, in modern buildings (about 20 years and newer) the opposite is more likely to be the case. Because of their importance to the structural integrity of the building, in newer buildings in NSW balconies are almost always common property with the lot owners allowed exclusive use of them.
However KP’s advice on the membrane is spot on (and applies to most balconies too). Again, check your strata plan and if you are still confused, get a strata lawyer to look at it for you. Once you’ve established what’s yours and what isn’t proceed as appropriate.