We did the Cape in 2015....Water is readily available most every where
x3.
I took 60L of water, but 2 kids and I lived out of 1 x 20L jerry, topping it up when we got fuel or had potable water at a camp. We drank the water out of the jerry and used creek water for washing and the like. The other 2 drums didn't even come out of the vehicle.
Fuel was similar, it is available most place, you will pay a bit more for it, but in the end it won't be worth the inconvenience of loosing space so you can wedge another jerry in. I took 40L along and used 20L of that once. We went from Wiepa to Chili Beach and then back to Bramwell Junction. I was looking at the map and the fuel gauge and thought better safe than sorry, I still had 25L in the tank when I got to Bramwell Junction. It could have been avoided if I had planned better and filled up at Lockheart River.
I have a ute, so space isn't that much of an issue, but if I was wagoning it, take 20L of each and top up the tanks when the opportunity arises.