And that is the reason why I get Australia Post to weigh every item I send using Click & Send or ebay Postage Labels. I lodge over counter (parcel not sealed) and request them to check the weights. If they are over I take home and repackage or do it at the counter. I usually try to keep a couple of grams under the maximum weight.

Have you incorrectly used a larger satchel (500g in a 3kg satchel) as you will be posting at a wrong rate.