There is no guide for applying your own shipping to the PowerStore but the idea is similar to changing out the payment gateway if you are using one of the supported providers.
Are you planning on using eCart to add in support for Australia Post? If so then you can just add the shipping quote server behavior to the confirm page and configure it to meet your needs. So long as you do this and have the other shipping options disabled then you should be able to use Australia Post instead of the others.
If you are not using eCart to add Australia Post then you will need to implement the shipping quote code for your provider in a custom fashion. This would require you to come up with the code and add it to the confirm page to get the shipping amount. You would then need to craft the shipping charge rule to work with the value you are getting back from your shipping quote.