YES. 🙂 Of course you can.
1. You just need to ensure you have international calls and texts enabled and allowed from your profile here: https://giffgaff.com/call-services
2. You also need to have airtime credit / PAYG balance as a goodybag does not cover international calls and texts.
This is the cost to call or text an Australian number:
I hope this helps! 🙂