Haven’t been connected yet, but have been tracking the NBN for MDUs for a while. The whirpool broadband forums on the NBN might be a more appropriate forum for your query. Link to the MDU thread @ whirlpool:
https://forums.whirlpool.net.au/archive/2131608
Your OC should not be up for any cost. The NBN will either run FTTP, FTTB or use existing cable infrastructure to connect your residents. When they’re ready to commence rolling out in your area your OC will be contacted. You can register your building with NBN Co & obtain a registration & LOCID if you want to be on the contact list.
More info here: https://www.nbnco.com.au/