1. What version of zencart and what version of PHP are you using?
2. What other modules do you have installed?

3. What version of aupost are you using? It will be displayed at the top of the screen in shipping modules when auspost is selected.
Click image for larger version.  Name: aupost_version_number.jpg  Views: 73  Size: 9.5 KB  ID: 20418

4. Is the user logged in or is it a guest user?
5. Does this occur when you add items to the shopping cart or when items are in the shopping cart and you enter an address and press the Update button?

Please provide all of the details requested above so I can assist.