I have found a simple solution that has finally worked for me. I have been struggling with this issue for a couple of weeks now, until i looked up the auspost "postage assessment calculator".

http://www1.auspost.com.au/pac/aus_parcel.asp

Auspost only calculates parcels with a maximum weight of 20kilos (20000 grams). (packages of over 20kgs are calculated as multiple packages). Realising this, I simply went into the admin area of my online store, went to "configuration", then to "Shipping/Packaging"
and then edited the section "Enter the Maximum Package Weight you will ship". I simply changed the the value from the default "100" to 20.

I have tested this with thousands of kilos nad all is working well now.

Hope this helps..