The Australia Post Shipping Extension does not have an option for free shipping built in however we suggest to follow the guide below to offer free shipping to your customers.
1. Set the freely shipped product to 0 grams in the Shopify product setup.
2. Setup a weight-based rate in your Settings -> Shipping panel to target carts which are 0 grams with free shipping.
Repeat this step on the zones you wish to offer free shipping on.
You will now have free shipping enabled.