An investigation into the ditching of a helicopter in Lake Rotorua in February 2013 has found the most likely cause was engine malfunction.
The pilot and passengers were able to escape uninjured and were picked up shortly afterwards by a nearby jetboat.
The Transport Accident Investigation Commission found oil had accumulated in the engine's right magneto because an oil slinger had been taken out during maintenance.
It also found the imported second-hand engine had not been subjected to the required level of scrutiny before being released to service.
The commission recommended the director of civil aviation give specific guidance to companies on the inspection of parts and components with unknown service histories.
It also recommended the use of quick-donning lifejackets for all occupants of single-engine aircraft when flying over water.