There had been rain on the strip and the surface was damp with muddy patches. A witness observed the aircraft become airborne after one half to three quarters of the strip length and climb to 5-7 metres above ground level. The witness looked away at this point but when he checked a short time later the aircraft had disappeared. The wreckage was located some 300 metres beyond the end of the strip in a tidal mangrove swamp. The pilot, who had limited experience in bush operations, reported that the first indication he had that all was not normal was when he crossed the upwind end of the strip at a height lower than expected. From this point on, the aircraft slowly descended into the mangroves. He did not detect any abnormality with the aircraft or its engines before or during the takeoff. An examination of the engines revealed that one and possibly both magnetos on the rear engine were not functioning correctly at impact. This would have reduced the aircraft's performance in the manner described by the pilot. The condition of the strip, coupled with the pilot's low experience level, could have reduced the prospect of him detecting a loss of performance much earlier than he did whether the magneto failure(s) occurred before or after liftoff. This occurrence was not the subject of an on-site investigation.