August is fine weather in Sydney. It is the rain that you should be careful about more than the cold. There are more storms as spring arrives and it starts to warm up (and worse weather for a tourist, in my opinion). If you want warmer, visit Cairns or the Great Barrier Reef. Think of Sydney like San Francisco and Cairns like Honolulu. I'd much rather be in Sydney in August than in Sept. or Oct, but that is just my taste (I think SF has a nice climate).