We rode in Australia in March, as the final destination of our overland-tour. There was a very unusual heat wave (for that time of year) around Adelaide and Melbourne, with daytime temperatures topping at +45 Celsius.
We had no extra space anywhere to put our riding gear, so I did the whole trip with a black leather-suit. Yeah, it was hot - probably wouldve been hot, if I´d ridden naked as well!! The worst problem for me was the feeling of falling asleep while riding in the heat, Im thankful that I didnt. Actually wasnt so bad, when you were moving, only when you stopped if you didnt find shelter immediately, that was a little annoying.
You´ve got to drink (A LOT!), in those conditions you can easily take 1 liter of water at each stop. Also keep in mind you will lose salt, etc., as you sweat, so just plain water all the time isnt the right solution.
But there had been hot conditions on our trip many times before, this couldve been the hottest by numbers, but all in all still didnt feel as intolerable as the humid, tropical heat in Malaysia or Indonesia.
It´ll be a whole different story, if your plan is to head into the more remote regions during the hot season. Thats when you will really need to be able to carry enough water and supplies, or you may end up in serious trouble.
There are fuel stations/roadhouses about 150-200kms apart across the Nullarbor, so not really a problem crossing it on the main road.