Hi Michael and Mel, generally most people go north for the winter and south in summer, could you go inland on your way up to Cairns and do some of the Coast. My inlaws travelled from Victoria up through the centre to Darwin, to kimberley, broome and down the West coast and back home in 3 months, thats with no kids, just for an example.
We have travelled through the centre and topend, all absolutely amazing, especially the kimberley.
Enjoy the trip, around Australia could take a year especially if you go inland, plenty to see there as well. Cheers