A helicopter that crashed into Lake Rotorua during a scenic flight lost power due to an engine malfunction, it has been determined.
The Robinson R44 Raven II light helicopter, which was operated by Volcanic Air Safaris Ltd, was also found to have an imported, second-hand engine which had not been properly examined before being signed off.
The chopper was on its second scenic flight of the day, with a pilot and three passengers on board, when it lost engine power.
Witnesses reported hearing "banging noises" from the helicopter, the Transport Accident Investigation Commission (TAIC) said.
The 30-year-old pilot managed to turn the helicopter towards the shoreline, and successfully ditched into shallow water near the lake edge.
All four people on board were able to get out uninjured and were picked up shortly afterwards by a nearby jet boat.
The TAIC investigation found the most likely cause of the power loss was a malfunctioning of the engine's right magneto - an engine component which generates the spark to initiate the combustion of fuel.
The malfunction was caused by a build up of engine oil, because an oil slinger, which was being replaced during a maintenance procedure, was not fitted.
During the inspection the TAIC also found the R44 had been fitted with an imported, second-hand engine and its associated records had not been subjected to the required level of scrutiny before it was released to service.
The R44 had flown 67 hours on the reclaimed engine before it crashed.
The helicopter's original engine had been due for an overhaul in January 2013, after it had racked up 2000 hours flight time.
"Rather than overhaul the engine, the operator decided to replace it with a second-hand engine sourced from overseas," the TAIC report said.
The engine had previously been installed in an R44 registered in Germany, which had struck the ground and another helicopter while landing in Sweden in 2011. It sat in storage for 18 months until sold to the New Zealand company, without the required release-to-service documentation.
It underwent a number of Civil Aviation Authority (CAA) checks before being allowed to fly. However, the TAIC said the company who carried out the inspection did not make attempts to confirm details of the crash or question the lack of information included in the engine's logbook.
As a result, the engine parts, including the magnetos, were not examined for damage.
Despite this, the TAIC said it was "unlikely that this safety issue contributed to the power loss".
In its findings the commission said the regulator's guidelines for inspecting used parts of unknown origin "were not well defined", and the company's approved procedures based on the guidelines "were not adequate in this case".
It also warned that the outcome of the crash "could have had a less favourable outcome" if the chopper had ditched into deeper water, as none of the people on board were wearing life jackets.
It recommended all helicopters should have "quick-donning life jackets" for such emergencies.