Walked the Carnarvon Great Walk in April this year with 5 others. From memory walk is closed 4-5 months over summer. In winter the nights would be VERY cold, but as a Melbournite, that might not be an issue

, April or September would probably be ideal. There's only one road into the Gorge & even if you flew from Melbourne, would still need a vehicle from Roma (no buses etc that I know of). 2wd vehicle is fine, unless heavy rain, which would be unlikely over winter.
Trip report, video, photos here:
www.danthewanderer.com