I have seen reports of issues like this and it seems to be related to session management. You can create a phpinfo page and check the session hander and save path to see if you have valid values for these. Also, there is a set of test pages that you can use to test out your session as well. This info can be found in post #20, here is a link back to it:
Please check on these settings to see if they are configured properly and also do a test of the pages to determine if you have sessions working properly. Please post back and let us know what you experience.
As for the ABN and GST you can alter the email template for the receipt and add in your ABN there for reference. The receipt should show the tax if you have set it up this way with eCart.