big place, temperatures and climate vary. But their summer is the UK winter (December or so onwards). Bear in mind that for the real hot places, you'd probably prefer their winter.
All depends where in Oz you intend to go...its a continent and a big one at that so many climates in the one country.
Brisbane best time to go is our spring or autumn.
Sydney best time to go is our winter.
Cairns best july or aug.
By the way if you are travelling across Australia - there is a brilliant company called The Wayward bus - - I used them to travel from Adelaide to Melbourne and it was one brilliant experience alontg the Great Ocean Road.