Hi. I am an Aussie from Victoria, so I'll just put my opinion in. In answer to your question about why not December to March. 1. It is VERY hot. If you go inland, it will get even hotter. Even in Victoria, we usually have days of up to 40 degrees C. 2. Bush fires. Big bush fires and fast bush fires. You do not want be stuck in the bush with a bush fire around you, with no phone service, limited maps and your GPS not working. So, try and stick to the main roads. . Other than that, enjoy your stay in Australia, it is a great place! And if your looking for birds, use eBird.org , we Aussies use it all the time, and it could help you to figure out where to go. And one more thing, if you have a bird ID question, use a facebook group called Australian Bird Identification. You'll get a response in less than a Minute usually. As you have probably guesses, I like birds!
Anyway, enjoy your stay!!