Whenever you build a duplex or development a NBN small dev application is required to create the address in the system.
This is because NBN allocate 1 connection per lot.
To get another connection you need to submit an application and pay the application fee(i think $600 per additional dwelling) .
In brownfield estates(existing ones) both premises pay the fee
Then after payment you go into a queue with all the other developments and wait for the job to be added to the system.
The time frame from application to acceptance is usually around 6 months so its best to do it at the beginning of the job.

Here is the link

http://www2.nbnco.com.au/develop-or-plan-with-the-nbn/new-developments/small-developments.html