There isn’t really a wrong time to visit Australia, it’s always nice somewhere (though hot doesn’t always mean nice). Officially summer starts in December, autumn in March, winter in June and spring in September. The sun-baked centre is more comfortable during the off-seasons (April-June and October-November). In the far northern parts of Australia there are only two seasons: the dry (May to October) and the wet (November to April), the more moderate dry season being the more bearable.
The south, especially Victoria and Tasmania, do get fairly miserable winters (and, with southern New South Wales, the snow). So, if you’re there for the long haul, a general rule is south for the summer and north for the winter. An itinerary might also take into account Australia’s fantastic array of festivals and prestigious sporting events. From Mardi Gras and WomAdelaide to the wonderfully preposterous Beer Can Regatta and the Grand Prix to the country-stopping Melbourne Cup.
Animal lovers should note you can gawp at majestic humpback whales as they migrate along the east coast from June to October and you can swim with gigantic whale sharks on Western Australia’s Ningaloo Reef from April to July.