The requires your store to have this feature Shopify Real-time carrier shipping enabled. This feature is included in some Shopify plans and you can have it added to your plan as well. Contact Shopify support to enable this.
If you have the feature Real-time carrier shipping, select our custom carrier for Australia Post (with Shopify Real-time carrier shipping) under Settings. Shipping and Delivery, Shipping, Manage Rates. As you define the zones you ship to, select the carrier Australia Post Calculator. See example below
Once you have the application installed to your store, fill out all the settings in the preference page.
In order to calculate shipping rate, you need to enter the origin of where the item is shipped from.
Review the instructions next to each setting to get more details.