Hi Nichola,
Could you turn on also, the debug mode for the module?
To do it, you need to edit the file helper.php in the folder /modules/mod_simple_australiapost_ecalculator.php and change the value of the line 144 from var $_debug = 0; to var $_debug = 1;
Save the file and upload on your server.
Also make sure, all your products have a weight in their description. Otherwise the calcul will be wrong.