I reckon that at least 500kg payload over and above the weight of the water is best - though it can depend on what equipment you carry. This is for single operator. I would rather err on the side of caution. If you add up the bits and pieces above the water weight you will see that even then I'm not allowing loads either.
Weight of driver. Let's say 100kg as I'm a porkie.
Weight of diesel. Up to 90kg when fuel tank is full (90 litre tank).
Weight of tank and (optional) filters. Varies a lot but I doubt mine is less than 100kg.
Hosereel(s) and hose. Not sure but maybe 30kg.
Poles. Not a great deal now I mostly use carbon but all the bits and pieces maybe 10kg (I carry an ali pole as a backup).
Backpack (with water) say 25kg.
Ladder on top for occasional use and stepladder inside for leaning over gates. Not sure but maybe 15kg.
That lot comes to 370kg.
I have an 800 litre tank. If it were full I would be fairly close to the maximum because my van payload is 1200kg (GVW minus vehicle weight) and a full tank of water would get me to around 1170kg.
However, although I have a 800 litre tank, 700 is the most I usually put in it or even 650.
There have been odd occasions when I have needed a full tank of water a long way from home (big job on a very long day). However, I have plenty of warning when such jobs are becoming due so I run down the diesel in the previous couple of weeks and unload any other equipment that I know I won't need.
So I reckon to be reasonably comfortable, only carry a weight of water that is below payload-500kg. Failing that, make damned sure that you unload anything unnecessary and run the diesel down if possible.
I don't think it's good practice anyway to constantly load a van up close to its maximum.