Not a question - I'd been googling this for a while and couldn't find a simple solve - and all the top results were from this forum, so I thought I'd post my solve here.
Typically Nest Wifi / Google Wifi plugs into the WAN port of your router, but if you have NBN HFC you have a separate ugly black box that plugs in there.
What I did was plug in my Nest Wifi into the LAN1 port, then while still connected to the existing network on my mobile, set up Google Wifi from the Wifi app with the same SSID and password as my existing network.
Then on the carrier provided modem/router was a 'wi-fi' button on the back, which I pressed to turn wi-fi off. My Google Wifi network worked! No bridging, no settings updates, and everything I'd already set up on that network (chromecast, home max) all connected automatically.
Let me know if this works for you!