OK, the problem is that something on your home page is causing a 302 redirect to that url you posted.
www.edmedia.com.au/home/language/en-AU/login.aspx works OK, as does clicking on the login link from the site root.
If I just request http://www.edmedia.com.au/language/en-AU/Home.aspx then I get the page back OK.
If I do a postback on http://www.edmedia.com.au/language/en-AU/Home.aspx then I get the 302 redirect to the above url (which appears to be some type of custom 404 handler page)
My suspicion is that something on your home page postback is causing an exception, and your web.config error handling is configured in such a way that it redirects to a broken link (the 404NotFound page). However, the Url Master part of the equation is behaving in the same way in both GET and POST circumstances. This is my suspicion - I can't say for sure I'm correct. Perhaps check your event logs and see if an error is being raised during that process, or perhaps temporarily disable your 404 handler to see what happens.
Note that if you're just going to support one language (en-AU) I would remove the base en-US language and uncheck the options to display the language in the Url. You'll just create duplicate content and hamper your efforts to rank better. Or just use the en-US language pack.
Finally, using the 302-to-404 page is not a good idea. What happens is that, when a page is not requested, instead of returning the correct 404 status back to a browser (or search engine), what you are doing instead is issuing a redirect to a page that says 'here it is'. If you have 404 errors, you should either return a 404 if it is a genuine error, or do a proper redirect if you have moved a page. You can decide what the balance is between telling search engines about your content and being helpful to users who follow an incorrect link.