Hmmm, I guess it depends what you are looking for. Of course Melbourne and Sydney are wonderful cities for a couple of days and there's loads to do but that's of course if you want a big city.
You could go to Darwin/central Australia, a more expensive option but a contrast to a large city and I think Tiger flys into Darwin from Singapore.
And then of course, there's Brisbane/Gold Coast/Sunshine Coast. In December/Jan it'll be hot and humid and the beaches and theme parks could be the focus. Would work well for 3 days. Further north could be too hot and it will be rainy season.
Since you've done the east coast you may well have seen these places and prefer something entirley different. One things for sure, australia has it all! rainforest, city, beach, desert
Guess it's a matter of the contrast you're looking for to the rest of your itinerary. I love WA but I think it's too far and too isolated for just 3 days......Nell may disagree!!
Have a think and if you want me to elaborate further on any place just say the word.
