I Have citroen dispatch 2.0 hdi, payload 900kg, 600l tank.
Put it on a weigh bridge approx 550l water (hard to fill tank 100%). Wooden ladders, spare battery, half a tank of fuel and me (just under 11 stone, don't know what my kg is). It was pretty much on the limit for the vehicle, front and rear axle loads were fine.
Tank heavier than average, it is 10mm polypropylene.
Ususally to be on safe side subtract 300 from payload to give you size of tank.
If your tank is a bit too big just don't fill it right up.
Payloads usually include the weight of the driver and fuel. Only way to check for sure is to fill up and go on a weigh bridge. Just gave the guy £10 cash as I didn't want the doccumentation.