you can use ecart to add Australia post.
first, open the pp_confirm page and delete the existing Shipping rate server behaviors for usps, ups and fedex, then go to the server behaviors panel and click the plus button.
Select eCart -> Shipping -> Australia Post.
Yes, you will need to have an account.
You would need to hand code the php to perform shipping rate lookup for an unsupported provide like tnt.
For Australia, you should probably use payapl Express checkout.
You can change the weight to kgs when applying the Australia post server behavior.