I'll try to help Rod out here.

Somewhere in the documentation accompanying the AustPost module, it explains that is uses a 'stack' method for multiple items because the coding was a challenge that Rod wasn't willing to take on. So, regardless of an item's area, the module will always stack one item on top of the other to determine the overall height of the parcel.

Note that I was discussing this issue earlier in this thread, as I have several products that can be stuffed into a 'large letter' size envelope, and mentioned a method/workaround for this situation. I have now implemented it and after some testing I am confident that it is working well.

Regarding the specification of product dimensions. Product dimensions (Length, Width & Height) are an additional to the Zen Cart database that are installed by the AustPost module. After a 'successful' and 'complete' install of AustPost, you should see these fields towards the bottom of the edit product page from Admin|Products... These are specified individually for each product. Where a product's dimensions are not specified, AustPost will use the 'Default Parcel Dimensions' for that product. If you have small products, you'll find that the default sizes will be far larger and will determine a 'parcel' every time.

Again, refer back to my posts early in this thread (and onwards) for further details of my 'suggested' workaround.

Specifying the dimensions for each product, if you have many, is very time consuming. For this reason, amongst others, I find Easy Populate an invaluable contribution. The only problem is that Easy Populate doesn't cater for the additional product dimension fields. I had to modify the Easy Populate code to accomodate these. I've now managed to do this and have it working very successfully.

I hope this info helps you out a little and may explain why you are getting these apparent anomolies with your Australia Post shipping estimates.

Cheers
GAM