I just got back from Sydney last Monday
The weather was gorgeous when I left it. I'd recommend taking your skates. It's a great city to skate around but it can be a bit hilly in parts. I did quite a bit of skating when I was back, though it was mostly slalom so I know some great slalom spots but I also went on a few street skating adventures. My favourite being skating over the Sydney Harbour Bridge. There is a pedestrian/bike path which you can take. Also, Centennial Park is quite nice to skate around.
There is a street skate every wednesday night. More details here: http://www.sydneybladers.com/
Enjoy! If you need any more info, just let me know.