We are working on a Shopify Application that will fetch Australian Post charges using it's API based on product weight , shipping container dimension and shipping to address. We believe this is a better way of charging shipping rather than pre-defining shipping zones which are subject to change.
You can sign up to be notified when it is done here https://webifytechnology.com/shopify/aus_post_app_signup.php
I know it's a very old post, but, I just wanted to update this post for the benefit of others with a similar query.
To calculate rates for Australia Post based on the weight & dimensions of your product, you can use the Australia Post Rates & Labels app. You can use the product dimensions along with advanced packaging options to calculate rates and handle shipping.