What type of NBN service did you get? FTTN/FTTC? If so, most of the Optus ADSL modems provided in the last year are compatible with the NBN, maybe 2 years. You might just need to plug it into your new set up.
you can have a look at the bottom side of modem
if there is NBN something, you can just connect NBN and Optus modem.
or if you're not sure you can live chat they can figure that out
you need a new modem or not. hope this helps.
I have the same issue, switched over but no modem arrived. It's two weeks overdue.
Managed to get my old one working, but performance is bad, helpdesk said I should use the new one as it would help with speed issues.
They still cannot find me a modem, have had two sent now but none turned up, but they did manage to send me a Toll bad with the installation instructions for the modem.