The basic idea is to be in the north in our winter (Jun-Aug) and in the south in our summer (Dec-Feb). I would suggest spending part of summer in Tasmania. It's full of wildlife and wilderness and quite scenic. I would suggest then spending some time in regional Victoria and Melbourne and then traveling from Melbourne along the Great Ocean Road to Adelaide, and then visiting Port Lincoln for the cage diving. From there you could head though the centre of Australia in Autumn for the outback, and then up to Darwin and Kakadu, arriving no earlier than May so you get the dry season. Head towards Cairns, doing your time in a cattle station while you're up north. Spend winter in the northern Territory and north Queensland, and then head down the Queensland coast towards the Gold Coast for spring. Then down the coast through Byron Bay on to Sydney, then home.

Depnding on what month you arrive you could modify this schedule to suit. You may want to fly some of the legs, it can be a very long drive. Obviously it's more fun if you do what seems interesting a the time, rather than following a strict schedule.