i am facing some big problem here . after filling all shipping address and hitting checkout button is leading User to error page by saying 'site can't be reached . I have have checked my error log , all are related to 'Australia Post'

here you go more information

URL : https://auspurifiers.com.au

Zencart version :1.5.4 php version in server : 5.6

This is what i found in log file:

0 /home/auau4207/public_html/includes/modules/shipping/aupost.php(307):
SimpleXMLElement->__construct('
1 /home/auau4207/public_html/includes/classes/shipping.php(140): aupost->quote('')
2 /home/auau4207/public_html/includes/modules/pages/checkout_shipping/header_php.php(190):
shipping->quote()
3 /home/auau4207/public_html/index.php(37): require('/home/auau4207/...')
4 {main} thrown in /home/auau4207/public_html/includes/modules/shipping/aupost.php on
line 307
why i am getting this error ? any help