If you wanted to add Australia Post shipping quotes to the PowerStore you would need disable the built in shipping support in the admin area under settings > shipping.
You would then need to add in the Australia Post server behavior to the confirm and pp_confirm pages. For the values in this server behavior it would be best to use the session variables that hold the checkout information like the shipping destination.
Once you have added this server behavior you will need to create a new shipping charge rule to be based on the Australia Post quote. When you are crafting the charge rule you would use the condition based on the Australia Post value being greater than 0, then for the amount you would just choose the Australia Post amount from the select list.
One more thing to keep in mind is that the shipping server behavior must be on the page in the rite spot. On the current confirm page the shipping server behaviors are on lines 446-666, when you add yours make sure it is around this same area of the page.
Please post back with any specific questions that you have about this process.