The pilot left his home base and flew to the strip from which he intended to conduct top dressing operations. Shortly after a normal take-off with the first load of superphosphate, the engine power suddenly deteriorated rapidly. The pilot dumped the load and landed in an adjoining paddock, but the aircraft collided with a fence and subsequently ground looped. No fault was subsequently found with the engine, which was still operating at idle power when the aircraft came to rest. After arrival at the agricultural strip, the pilot had left the engine idling for several minutes with the carburettor heat selected to the cold position. Atmospheric conditions were suitable for the formation of carburettor icing, and it was most probable that this had occurred. The pilot had been in the habit of using reduced power for take-off, which may have aggravated any tendency for carburettor ice to form.