Australia has a distinctly mild climate and many parts of the country are perfect even in the middle of the year, the Australian winter. When it is winter in the northern hemisphere, Australia is basking in the southern hemisphere's summer, and vice versa. Remember, unlike in the northern hemisphere, the farther south you travel in Australia, the colder it becomes.
The peak travel season in the most popular parts of Australia is the Australian winter. In much of the country – Queensland, the Top End, the Red Centre and most of Western Australia – the most pleasant time to travel is April through September, when daytime temperatures are 66° to 88°F (19°-31°C) and it rarely rains. June, July and August are the busiest months in these areas.
On the other hand, Australia's summer is a nice time to visit the southern states - New South Wales, Victoria, South Australia, Western Australia from Perth to the south, and Tasmania. Even in winter, temperatures rarely dip below freezing, and snow falls only in parts of Tasmania, in the ski fields of Victoria and in the Snowy Mountains of southern New South Wales.
Ultimately the best months to visit Australia are September and October. During this period it's still warm enough to hit the southern beaches, but also cool enough to tour Uluru (Ayers Rock). As well, the humidity and rains have not yet come to Cairns or the Top End and the wildflowers are in full bloom in Western Australia.
In addition to the period from December 26 to the end of January when Australians take their summer vacations, Easter (from Good Friday to Easter Monday) and all school holidays are very busy.