That won't work in Australia. You can buy the SIM anywhere of course, you just can't activate it. You'll need proof of ID on line to do that.. There are only three operators (the rest are resellers). I am sure there are ways around it but not the one you suggest.
When I was last in NZ you didn't need ID must buy a SIM and good to go, not sjre thats still the case though?
The twist they've introduced in this article is they're using the registration lock feature, which means you have a signal pin enabled, so as long as the account doesn't go idle for 7 days even somebody who gets the phone number can't use signal.
The fundamental problem is the signal foundation sees the phone number as the identity. If you don't have control of the phone number, you don't really have control of the identity.
The good news is, they let you change your phone number and maintain your contacts. But if the phone number the account is currently registered to get assigned to somebody else and you don't change it, then you're playing the 7-day roulette
It's fine for a temporary signal account, but if you let the number expire, then someone else gets assigned that number, and that new person wants to use Signal, they'll get your account.
They can't see your old messages, but they'll get any new ones instead of you.