There was a thunderstorm in the vicinity of Koolan Island when the aircraft arrived, and the pilot held until the storm had cleared the area. The pilot made a normal full flap approach touching down approximately 300 metres into the 661 metre strip. After approximately 100 metres of ground roll on the wet gravel/coral strip, all braking effectiveness ceased. The pilot attempted to steer the aircraft to the left to avoid overrunning the end of the strip, however, the aircraft skidded, turned through 180 degrees and came to a stop partially off the strip, resting on the cargo pod and the right wing tip. After the aircraft stopped the pilot observed the surface was hard, there were oil soaked areas on the surface and water was still running off the strip.