Hi
The built-in cellular features require a carrier plan for your iPhone that specifically includes support for Apple Watch, including the carrier activating the device on the plan.
Amaysim is not currently listed as a supported carrier in Australia:
Apple Watch - Carriers - Apple
I suggest that you contact your preferred carrier to ask whether any given model can be used on their network, now or later, with any service plan that they may offer or plan on offering for iPhone that supports Apple Watch Series 3 (GPS + Cellular).
You may also wish to monitor the Carriers page for changes over time.
If your preferred carrier does not currently support the built-in cellular capability on Apple Watch Series 3 (GPS + Cellular), then the model can still be used like a GPS-only model, without activating the cellular capability, which can be enabled later.