In truth, it is hard to say. You may need to be flexible in your destinations. If you are then I would I would pick October. Earlier the better. July *can* be bad for cold stream weather. August usually gets very stong westerlies. November can get nasty weather in terms of active fronts. Actually, so can October but it isn't as hot.
Pop over to the Bureau of Meteorology site and have a rummage around there. I *think* it is
www.bomw.gov.au Good luck.
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reddo...feral animal!