Same issue here. I'm with Internode ISP so not isolated to Telstra and iiNet. Tried lots of things including buying a new router as I updated the firmware on the old one prior to this issue. Since finding out others have been having the same problem around the same time I will stop pointing to me as the fault (slowly bending finger towards Google/Spotify)
Last update from me. After taking to Twitter, Spotify support advised to try a re-link of my account again using the Google Home App. This appears to have now fixed the issue. While I had done this previously, it is possible they resolved an underlying issue that the re-link will fix.
If you haven't done this in the last few hours and are still experiencing the issue, perhaps give it a try.
Open your Google Home App > Devices > Google Home > Music > Un-link
Link to Spotify > Enter credentials > Done > Test.
Good luck everyone; I hope this is the last we see of this issue.