Although it’s not active it next month in Australia, you can get it to work by changing your location on your sonos account to United States. No VPN is needed nor does your google account need to change locations. Just make sure your sonos is up to date, and select google assistant in the voice services.
Change your location here: https://www.sonos.com/myaccount/profile/
At this stage I think you have to manually install the updated app on Android, but on iOS you are good to go updating from the App Store. I don’t have an android, that’s just what I’ve read online, so it may have changed.
Is there any other ramifications of setting your location to the US?