Hey Monkeykiter,
Sydney is a bugger in winter, apart from the cold and the wind chill, the sou' westers are cold and very gusty. The westerlies are even gustier sometimes going from 10-35knots and back within seconds, it makes for some pretty unpleasant and potentially dangerous kiting.
kurnell is one of the only places you can kite in Winter in Sydney, also Boat Harbour/Merries reeefs and maybe also Pittwater on the North side. None of these places are very secure or safe for learners so if you do persist as you probably will, take enormous care in the westerlies and make sure you know how to activate your safety system.
I was almost killed twice at Kurnell in Westerlies,

once on land with a massive lofting in a big gust (luckily wasn't hooked in and just jarred my back upon return to earth) and the other time kiting in Quibray Bay behind Kurnells Bonna Point. (Got yanked hard in a gust and flew approx 40 feet downwind. I missed an oyster encrusted pole by 3 feet.

My kite luffed and fell onto a rock covered in oysters and was destroyed. I was actually returning to the beach to pack up after a few earlier scares when it happened)
Know your limitations, check the forecast and ask the locals what they think before venturing out. Or just go to Port Douglas!

Good Luck
Darren Marshall
Australian Kiteboarding School