it could be that the service type selected does not support shipping from australia to new zealand,
add the following code to the failure page so we can see the full request and response coming back:
Request:<br />
<?php echo(htmlentities($_SESSION["bakatus_AusPost_FullRequest"])); ?>
Response:<br />
<?php echo(htmlentities($_SESSION["bakatus_AusPost_FullResponse"])); ?>
I will forward the info about the api changes to the development team so they can look into it.