Depending on which part you visit (it's a b-i-i-ig country) Wintertime in Oz is pretty warm. I've swum on the Barrier Reef in mid winter and yet, there are places we can get snow. A couple of English friends visited one winter and couldn't believe how pleasant the weather was.
Just at the moment we're in the middle of a wet spell. I'm vacationing about 200 km south of Sydney at present and today is the first time in almost a week when it hasn't rained.
So it depends on where you're planning to go and what you plan to do.
I live in Australia. In country Victoria. You take your chances with the weather. We have just finished Summer & beginning Autumn but that doesn't mean a thing. Bring a warm jacket but be prepared for beautiful sunny days as well. There is beautiful countryside bed & breakfasts to stay in & the cities will fill all your shopping needs. The food is good & the people are friendly. There are areas where you won't want to leave cos of the beauty & tranquility. Stay along rivers & enjoy fishing walking tracks etc. anything you want to know about Victoria just ask.
My mom went to Australia on her 14th birthday. She said it snowed for the first time in thirty years. She went on a boat from there and crossed the International Date Line. So, technically, it was her birthday for two days.
People are friendly, weather is mostly very nice. Some things are in the opposite way because we are in the Southern Hemisphere. Also right hand traffic like in the UK. There are the infamous snakes and spiders but you're most likely to see none on a trip. Distances are huge and there is a lot of empty country called the Outback. South east is most crowded. Some of the most popular places to visit are Sydney, Melbourne, Gold Coast, Great Barrier Reef, Uluru (Ayers Rock). http://www.gondwananet.com