How about email sign-in on web page?
You can create an email sign-in on the web page, or sign in with F or G+,
but no option to sign in with existing email account.
The reason why it does not allow email sign in is because that page is mainly used for new users to download the program CAM.
pekson scott commented
When you are not unable to setup BigPond on iPhone, so you can get in touch with BigPond customer service whenever you want to clarify any doubt related to the setup process.