Can you use 2 phone numbers in Australia similanteously with imessage

Yes, you can have two number for Messages. Although I've not done this and can't test it I have read several comments - such as this one - from those who were successful at setting this up.

Add or remove your phone number in Messages or FaceTime describes how you can add a phone number to your Messages app from the device, and use it thereafter.

Check also Settings > Messages > Send & Receive in the second phone and select your iCloud account name in the two boxes (Receive From and Start From) before you try to add the second number. It might also work for you.