Do you have the source code? You can see the calculations in
...\src\Plugins\Nop.Plugin.Shipping.AustraliaPost\AustraliaPostComputationMethod.cs
Basically,
it estimates the # of packages based either on sum of a dimension (largest sum of either H, W, L) or sum of weight for all items.
Dimension sum is divided by 105cm
Weight sum is divided by 20kg
Whichever is greater is number of packages. Then it takes above sums, and divides by # of calculated packages to recalculate
(estimate) per package dimension and weight.
IMHO - Not a very good algorithm.
I've discussed it elsewhere in these forums (Google for "dimensional weight")
Shipping Director does packing, and has various
Rate Request Types - if you set $ShippingRateRequest to "OneRequestPerPackage", then SD will make multiple calls to the carrier to get per package rates, and then sum the rates (for each method) to present a single rate to customer (per method - e.g. ground, air, etc. - whatever you configure in the other shipping plugin configuration).