What type of NBN technology do you have? If it's an NBN FTTP service, then you just need to connect the main router to the NBN NTD box replacing your old router. Once done, configure the main router with PPPoE connection type and key in the ISP username and password.
Internet should work once done.
On the other hand, we have customers how uses Google nest with their NBN HFC. Check these threads: Google WiFi, TPG HFC Modem and Google NEST (not Wifi), Connecting Google Nest.
