Well, much study was done in Sydney re weather for the Olympics. It was decided that September is the go. So, I would suggest September.
Thunderies are reasonably predictable in Sydney. Usually they appear in the afternoon in Summer and or along a cold front (aka Southerly). Vis is generally pretty good if there is a high pressure system over NSW. May be a tad hazy but over all excellent.
It depends on what sort of flying you are looking at doing. However, September generally is the best month for most of Oz. You will get snow and ice if you head south (ie our snow season is peaking usually in Sept) but if you go either west or north the weather is beaut.
Good luck.
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reddo...feral animal!