Depends where you're heading. Queensland is very mild all along the coast and only requires light clothing. Darwin in Northern territory will feel like summer. If you venture to Melbourne in Victoria, you can experience all the seasons in a day-very changeable but you'll require warm clothing. Tasmania is cool even in summer and very cold (by Australian standards) in winter. June/July are good times to visit northern Queensland and Darwin as the cyclones have passed and the weather is not so hot and humid. Best time to visit central Australia and Uluru-Kata Tjuta National Park. It's a big country so the temperature varies dramatically.