I heard from several sources that what they do is give the apartment builder a paper bag of cash in exchange for the right to use their wires instead of the NBN. Then they gouge the users with higher monthly fees.
When I was there NBNCo hadn't really moved into the inner city yet. We did have some kind of financial agreement with the building developer/management to install our VDSL DSLAMs in their comms room. It wouldn't surprise me if those payments got shadier and more aggressive as the NBN coverage increased.