Spring brings warmer weather to southern parts of Australia, but temperatures are only a few degrees Fahrenheit warmer than in August, on average. Sydney and Perth, on roughly the same latitude but on opposite coasts, experience average highs of 68° F (20° C), which is pleasant for city sightseeing activities. Melbourne and Adelaide are a little cooler, with average highs of around 63° F (17-18° C). Canberra (inland) and Hobart (on the southern island of Tasmania) stay quite cool, with highs around 59° F (15° C).
The most noticeable shift in temperature from August to September occurs in the desert: in September visitors should expect hotter days, but still relatively cool nights. For example, Alice Springs has an average high of 80° F (27° C) in September, but an average low of 50° F (10° C). So, bring layers, and expect to shed them as the day warms up.
Rainfall is low throughout the country, even in cities like Sydney and Perth that see heavier rainfall throughout the winter. However, although September is still the dry season in the tropical north, this month is not as dry as June-August, so visitors might experience building humidity in September.
Crowds and Costs
September is shoulder season in the tropical north, as rainfall is low and conditions are generally good for swimming in the sea (i.e., there's less risk from deadly jellyfish), but southern Australians are less likely to be escaping the cold like they do in winter. In popular places like the Whitsundays, accommodation may still book up fast, so make bookings well ahead of time if you have any special preferences.
Big cities like Sydney and Melbourne get visitors year-round, but September is not an especially busy month. You're likely to find plenty of accommodation options throughout the south at non-peak prices.
Where to Go
Where you decide to go will largely depend on the activities you want to do, and the kind of weather you like. But, there's hardly a 'bad' place to be in Australia in September.
If you don't mind cooler, sometimes-wintry temperatures, then Tasmania and the south-east of the mainland are good options. Just don't expect to lounge on a beach and get a tan. If you do want to lounge on a beach or do some water sports, then northern Queensland and northern Western Australia are great options (northern Western Australia is much more remote, thus less busy, than Queensland).
If you want to do some big-city sightseeing, then the conditions in Sydney, Perth, Melbourne, and Brisbane are good for inside-outside city activities. If you want to experience the Australian outback, conditions are good for visiting Uluru, Alice Springs, and the national parks (Kakadu, Litchfield) of the Northern Territory in September.
What to Do
As well as those mentioned above, if you're into skiing, Australia's ski season runs until early October. Although not necessarily famous as a ski destination, the mountains of Tasmania, New South Wales, Victoria, and the Australian Capital Territory have ski fields of around 5000 feet (1500 meters) in altitude that offer good conditions.
September is a good time for a long-distance road trip, are conditions are suitable for long periods in the car (ie, not too hot). Depending on how much time you have, you can drive the Great Ocean Road south of Melbourne, or along the northern New South Wales coast between Sydney and Brisbane. If you have a lot longer, then Adelaide to Perth, Adelaide to Uluru/Darwin, Brisbane to Cairns, or Brisbane to Uluru are amazing options.
Flower and garden shows. Keen gardeners and flower lovers are in luck if traveling around Australia in September, as there are a number of flower shows and festivals to enjoy. Canberra's Floriade lasts for a whole month, usually from mid-September, and shouldn't be missed if you're visiting the capital.
Travelers to Western Australia can check out the month-long King's Park Festival in Perth, with a variety of walks, art, talks, and plant-related attractions. The town of Bowral, in New South Wales' Southern Highlands, puts on the Tulip Time festival in September, with 90,000 tulips in bloom. You're likely to find other spring festivals throughout the country.
Traveling to Australia in September? Check out these great itineraries
Ultimate Australia Self-Drive Tour - 20 Days. Explore the East Coast of Australia at your own pace on this 20-day self-drive tour. Do a circuit of the country that takes in the southeast, the east coast, and central Australia.
Exploring Northern Australia - 9 Days. Explore the natural beauty and diverse wildlife of the northern half of Australia, making your way from tropical Cairns to lively Perth on this 9-day trip.