Any time is a good time to be in Australia. Summer (December to February) can get uncomfortably hot but it’s great beach weather. Up north, the summer wet season is very, very humid and the sea is swarming with box jellyfish. Winter (June to August) offers skiing in NSW, Victoria and sometimes Tasmania. In spring and autumn the weather is mild.
Summer (December to February) can get uncomfortably hot just about anywhere, even in Tasmania. If you’re in the southern states during these months it’s great beach weather and great melanoma weather. Up north, this is the wet season, and it’s very, very humid (you’ll need to check for jellyfish before jumping into the water). On the upside, the Top End is beautifully green and free of tourists at this time.
From June until August things in the north have cooled down a little and dried up a lot. This is a good time to visit Queensland or the outback. If you’re here for the skiing, now’s the time to head for the snowfields of NSW and Victoria.
Overall, spring and autumn are probably the safest bets the weather is reasonably mild wherever you are, and spring brings out the wildflowers in the outback, while autumn is particularly beautiful around Canberra and in the Victorian Alps.