I'm pretty sure that you have to maintain the credit average for the 4 years. Which is marks of 65% and above.
I don't do orthoptics so I cannot say whether it is going to be hard for you to maintain a credit average because I don't know how hard you will find the content of the course.
Essentially it's the same course as the Orthoptics except that you graduate with 2 degrees and you will be studying it at a Masters level, which will probably be more demanding. But, it's a higher qualification, so it has its benefits.
You probably know all this already, but if you want to email the campus with any questions (they're probably more qualified then me to answer, I don't wanna lead you in the wrong direction.) Use this link
http://www.fhs.usyd.edu.au/future_students/emailenquiryform.shtml
And you've probably seen this too, but:
http://www.fhs.usyd.edu.au/future_students/undergrad/new_progs/bhs_clinical_vision.shtml
If you're keen, but not completely sure, come to the Information Day on Jan 3rd. I'll be there and so will all the other ambassadors and my superiors who will DEFINETLY be able to give you any more info you want. I also have a friend who's in the current orthoptics program, if I see her, I'll ask her about the workload in the undergrad course for you. And she's an ambassador too, so she'd talk to you.