Sydney is fabulous all year round! Winter is generally mild but if you do strike a cold and wet patch there are dozens of indoor activities and day trips that will keep you busy.
The beaches around Sydney are of course best between October and April when it is almost always warm enough to swim. Futher north, around Byron Bay, the weather can be glorious all year and the brave will be willing to swim even in the middle of winter.
If you want to be active in the Blue Mountains then summer is usually more comfortable but the winter months offer the prospect of cozy fireside nights and deliciously warming wine & food.
Inland, on the edge of the Outback the summer months are hot & dry while the winter months are mild with plenty of blue sky days.