The pilot was carrying out a cattle-spotting operation prior to the commencement of the muster. He descended the aircraft to 500 feet above the ground to get a better view of some animals amongst the trees. The pilot reported that as the aircraft was being returned to level flight it was affected by severe turbulence and the pilot's door became unlatched. While attempting to close the door he noticed that the indicated airspeed had reduced and that the aircraft was continuing to sink. A turn to the right, to clear the area of turbulence, was commenced but during the turn the aircraft stalled and entered a spin. Attempts to recover from the spin were unsuccessful and the aircraft struck the ground in a nose down attitude while turning to the right.