we would need to see the full request and full response being sent to Australia post.
add the following code to your page to return the full request and full response from Australia post, this should help identify why it is failing:
<?php if(!session_id()) session_start(); ?>
Full Requesst: <?php echo $_SESSION["eCart4_AusPost_FullRequest"]; ?><br />
Full Response: <?php echo $_SESSION["eCart4_AusPost_FullResponse"]; ?><br />
NOTE: you will need to change "eCart4" to the name of your eCart object.