First I should explain what the GVM is. The GROSS VEHICLE MASS of a vehicle is the maximum all up weight of a loaded vehicle including fuel, people, and goods etc. All vehicles and models of the same vehicle can be different. The GVM for your vehicle can be found on the Manufacturers Plate on the fire wall in the engine bay.
The amount you can carry depends on the weight of your vehicle to start with, the lighter your vehicle the more you can carry. For this reason it is best to consider fitting lightweight accessories to start with e.g. aluminium Bull Bars and Side Steps instead of steel, aluminium trays and toolboxes etc. instead of the steel equivalent.
Because the vehicle will be converted to something totally different it is impossible to say accurately just what the finished vehicle will weigh, but I can give you a guide as to what to expect.
A Standard 100 series Diesel Wagon weighs approx 2300 kgs empty. If we cut the rear off and replace it with our Dual Cab wall and extend the chassis, the weight should end up about the same. So you can see if you subtract 2300kg from the GVM on the manufacturers plate (in this case 3200kg) you will have a carrying capacity of 900kg. Keep in mind you don’t have a tray or people on board at this point, so this is where you have to make up your mind as to whether to fit a Steel (approx 240kg) or an Aluminium tray (approx 125kg).
It is possible in some states to have a GVM upgraded by an engineer on most vehicles by using different springs and shocks etc. The amount you may be able to upgrade a GVM varies between models . Talk to your local State Dept of Transport for more specific info.