Quite impossible to answer. There are always great events which are missed out. In October/November we were too late for the humpback whales in Hervey Bay, too early for the turtles coming ashore, too late to see Kings Canyon before getting burned and too early for a big Darwin thunderstorm.
But we were absolutely right in time to see the Jacarandas flowering, to see friends one doesn't met home very often, to see a lot of birds and crocs in Kakadu, not to get cold in the centre. It was the right time as there probably is always a best time to visit for some part of Australia. There is only one thing I would strongly recommend: Unless you have a really good reason, do not travel the whole North-South range at one time. I don't think I normally carry too much luggage, but this time it was far too much. Warm gear for Tassie and it's 5°C with sharp winds sometimes, the dive suit for Queensland and the suitable nothing for the 35° - 45° C at the top end. I was aware of the problem before, but when spending a lot of time and nerves dealing with luggage one doesn't need most of the time, it's really stressing. Leave that, if you can.