The Rates you set for your products in Shopify can either be free, flat-rate, or calculated. Calculated rates are those that have been provided by a third party app, such as ours.
Australia Post Shipping automatically displays calculated rates provided by Australia Post to your customer, which means you don't have to manually enter in each rate or update them if Australia Post's prices change. All you need to do is make sure you have enabled Australia Post Shipping for each of your zones. Learn how to set up you zones and enable Australia Post Shipping rates at the same time.
To find out more about rates, Shopify has written a good article about how they work. Scroll down to 'Create calculated shipping rates' if you've unsure of where to start.