The East Coast is great in December, don't listen to the blowhard, Kitehard!
Its definitely windy on the East or Best Coast in December, Sydney is good for kiting, but the spots right in the city are limited to one large flatwater Bay, being Botany bay, and then the rest of the spots are open ocean. Ocean means waves, which most people like these days.
Within 1-2 hrs drive either side of Sydney are awesome locations and beyond is just some of the finest on the planet. Perth and surrounding areas are good too, good mix of flat water and surf, probably some of the best anywhere, with strong reliable sea breezes over summer.
Summer in Oz runs for 4-6 months, October to March, ish.
Summer water temps on the East Coast are 19-23C, on the West Coast is is cooler, sub 20 degrees.
Lessons are one way to get a ride for sure, hardly anyone rents kites, way too expensive to insure and risky for the operator. Boards and other stuff no worries.
Unless you really feel the cold all you need is a thermal rashie and boardies, or maybe a shortie wetsuit, cost here is approx $100 -$250 for a wetsuit.
Forget NT and Darwin for kitesurfing in summer, same with anywhere north of Bundaberg in Qld.
The total Oz coastline length is just shy of the diameter of the Earth, the place is huge.
Cya and
Goodwinds
Steve