It is an acronym for the National Broadband Network. A government owned enterprise which was supposed to build a ubiquitous broadband network covering Australia but has been plagued with political meddling and interference since the get-go.

My residence now has NBN as an option. I have decided to wait a year or two. Going for NBN is a bit of a gamble. My OK-ish ADSL connection is finally working without interruptions, and on forums people with NBN are talking about resetting their modem several times a day, coaxial cable connections need to be perfect otherwise it doesn't work etc. Sad really to miss out, but I am prioritizing uptime over speed. The UK got this right in the late 90's by connecting cable to almost every residence (IIRC).