It's because the vcsweb server is down - try it yourself:
http://austpost.vcsweb.com/
My monitoring systems started alerting me at around 3:00-3:30 PM Eastern Australian time that it was inaccessible. I wondered since setting up a store for a client a few months ago just how reliable this server would be, and now I know.
In the meantime I've changed the server address in austpost.php to the direct
http://drc.edeliver.com.au/ Australia Post server and have hard-coded the "fixed price" rates in so that it can continue to function until austpost.vcsweb.com comes back online.
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