Does your carrier allow you to turn off roaming? If so, you can do that. Wifi calling will still work.

I use a pre-paid provider in the US, and whilst they do allow roaming, you have to put credit into your account for it to work, so I simply don't do that. No roaming means no accidental roaming!

Australian prepaid mobile plans are relatively cheap now days, however they are not good for roaming. They either don't support it, or if they do it's generally expensive enough to make it not worth doing.