Hi Laura
Our winter, their summer is probably the best time to go, though there are areas that have reasonable weather all year round. I have visited in both October and February and the weather was lovely though a bit cool in the very far south in October.
As you know it's a huge country so you need to tell us a bit about what you are interested in doing and how long you will be able to stay in the country. Are you interested in night life or wild life, cities or countryside or both? There are so many possibilities. There are lots of HT members who have visited Australia and I'm sure would be willing to give help and advice.
Sydney is probably the main city to head for with lots to see and do though other places shouldn't be ignored. Lots of people head for Cairns for a trip out to the Great Barrier Reef and Alice Springs/Ayers Rock for the Red Centre.
Judith