Hi,
As far as I can see on the screenshot of your HikaShop, it says that you have 6 packages and not one while on the Australia Post website it seems that you only asked for the shipping price of one package.
You need to test with the same settings if you want the same result. I guess that if you test with only one product, you'll get the same result.
Remember that the Australia Post shipping plugin is only giving you the shipping price based on what Australia Post told him it should be. So if there is a difference between what the Australia Post website says and the Australia Post web service says, it means that the settings for the calculation are not the same.