Well, a friend of mine got a UAI of 98.2 and studied econ last year.
He got a distinction average first semester, and slightly above a D average second semester.
He didn't get into law at Usyd.
UNSW did let him in though, so he is now there.
I had another friend who is doing full fee paying law at Usyd with a UAI of 97.6 and got a D average and they didn't let him transfer into com supported.
So unless you are getting predominantly HDs it seems that your UAI would have to be above 98.5