Hi,
In that case, you will have to configure the product as a pack of 12 bottles and you could configure a custom field of the table product via the menu Display>Custom fields so that you could enter the price per bottle as an information on the product page.
Otherwise, it would require modifying the code of the Australia Post shipping plugin to divide by 12 the quantity of each product in your shop but then you would loose the change after each update or you would have to make a copy of the plugin for your changes.
But doing all this would require a developer to do that for you.