ECG on Australian apple watch

If you purchased the watch for the ECG, you will not be able to use it except if purchased in and used in one of the countries listed here. watchOS - Feature Availability - Apple

You would have been better off purchasing in Italy. An Italian Watch will work with the ECG. Australia is not supported yet.

You would have to ask Apple that one. It may, but when you are in a different geographic area, you may not get the correct iOS update. However, it seems the model for Italy and Australia are the same, so I'm guessing it should work. Do you have the correct iOS on the phone, and watchOS on the watch? Check this, Taking an ECG with the ECG app on Apple Watch Series 4 - Apple Support