It's actually pretty hard to say. Since at USyd it's 70% coursework, your 3rd year results are somewhat reflective of what you might get in honours. However, it is certainly possible to get a 65 average and barely get in but end up with 1st class (but not uni medal), but that's rare. I'd say at least distinction in 3rd year, unless you somehow miraculously improve, or screwed up previously.
With predominantly research honours it's unpredictable. Some people can get HDs in earlier courses but have no ability in creating new work, while some might be brilliant at it but have poor WAM from previous years.
And for those of you aiming for uni medal, you'll need a 80 or HD average in 3rd year, and 95+ in honours. Usually this will guarantee you some scholarship overseas. (Hence, if you look at the honour boards, why so few medallist stayed on in Australia)