Australia is a large continent, so weather conditions vary greatly from one side to the other. However in general, summers are warm to hot and winters (May to September) are mild. Snow falls only on high ground.
Inland, much of the country is desert or semi-desert (the famous Outback). Summer days can be intolerably hot but nights are cold, especially during winter. The far north, including Kakadu and the Daintree forest, is lush and tropical because of the drenching rains and humidity of their summer - so it's best to visit these places in the Dry season from May to September, when the skies are a clear dazzling blue and the air is warm.
Most Australians live on the coast because the climate is at its most pleasant there, ranging from tropical to Mediterranean as you travel south. Much of the south coast has a more European (i.e. unpredictable) climate - in Melbourne there is a saying, "if the weather doesn't suit you, wait 10 minutes".
If you are planning a working holiday of longer than 6 months, don't make the mistake of leaving all your warm clothing behind - you will acclimatise after a few weeks of 30 °C heat, and a balmy evening will suddenly feel quite chilly.