You do not need to worry about coding the shipping quote code, it is handled by the eCart server behavior.
All you need to do is open your confirm page and go to Server Behaviors > eCart > Shipping > Australia Post.
Once you configure and apply this server behavior it will create the shipping charge rule for you. Once this is in place you will need to do some testing to ensure it is working correctly. Don't forget to disable the default shipping for PowerStore.
Please post back with any questions that you have about configuring or applying this server behavior.