You are able to enable and disable Australia Post shipping in any of your defined shipping zones from within your Shopify shipping settings.
Navigate to your Shopify admin and then through to Settings.
From within Settings, you need to open the Shipping section.
Within your Shipping settings, you can define which countries you allow your products to be shipped to. You are also able to define which shipping methods each of your shipping zones have access to.
You can add or remove access to carrier rates within this view to enable and disable Australia Post shipping to specific zones.
To add or remove a carrier from a zone simply click the edit link at the top right of your desired zone.
Once you are within the zone you can scroll to "Calculated rates" and remove the Carrier by clicking the X icon. Alternatively, if you wish to add the carrier to a zone (if it doesn't appear already) you can click the "Add rate" button and selecting the option.