What does package quantity refer to?
The WebAssist description (ecart_help.pdf) says ...
Package quantity: Sets the number of packages used for the shipment. If items are sent individually, this can be set as 1. Otherwise, you may wish to use dynamic data to determine how the shipment is to be broken up if there is an option for multiple packages to be sent.
Australia Post (#Data) says ...
Quantity: This is the quantity of items for which the customer is estimating the delivery charges.
I can get it to load up a maximum weight of 20kgs (if it's over), that's easy enough, but I would have thought that Australia Post would have been able to do a calculation of more than 1 package.
Got an idea though. If I divide the weight by the quantity so it sees it as 2 boxes of 12kgs in each box, could that work?