The flight was intended to provide familiarisation for the pilot on the aircraft type. After loading water into the hopper the pilot took off and carried out a series of turns before positioning for a spray run along one of the flight strips. At the end of the run the aircraft pulled up steeply and began banking to the right. It then appeared to enter a spin to the right and subsequently struck the ground in a steep nose-down attitude with little forward speed. The pilot had been instructed to load 100 gallons of water for the purpose of this exercise. Unsupervised, the pilot loaded 200 gallons of water which resulted in the aircraft being operated at the maximum all up weight approved agricultural overload. It subsequently stalled in a procedure turn at a height from which the pilot was unable to effect recovery before impact with the ground.