Thanks guys for all the insight,
another option is to take an "OZ Bus Tour" from Darwin down to Melbourne. The transport and lodging are included and it seems to provide a way to see things, do some hiking, some bush camp and then get to the south where it'll hopefully be cooler. I'm also adding an extended stay in Tasmania to do some hiking with the aid of a rental car...

This is good thinking as it takes a lot of the time pressure off as one tries to link in transport options.

"where it'll hopefully be cooler"
You are right to say 'hopefully' as it is often hotter in southern and Western Australia in summer than in the north east which almost never gets the hot, dry continental winds.