The pilot of an aircraft that crashed near Raglan, killing him and his wife, was on medication for an anxiety disorder and probably should not have been flying the plane, a final report into the incident has revealed.

2degrees boss Eric Hertz, 59, and wife Kathy Picone Hertz, 64 were killed when their twin-engine Beechcraft Baron plunged into the sea during Easter weekend in 2013.

In the final report into the investigation, the Civil Aviation Authority of New Zealand found there were multiple contributing factors which caused the plane to lose speed and enter a spin it could ultimately not get out of at about 12.20pm on Saturday, March 30.

The reason for the aircraft rapidly losing speed and disappearing from radar was inconclusive, but the most likely reason was it experiencing a reduction in power or power loss from the aircraft's left engine which caused it to crash near the Kawhia Harbour.

The wreckage of the Beechcraft Baron twin engine aircraft that claimed the lives of 2 Degrees CEO Eric Hertz and his wife Kathy.
The wreckage of the Beechcraft Baron twin engine aircraft that claimed the lives of 2 Degrees CEO Eric Hertz and his wife Kathy.

The couple's family has thanked the CAA for their efforts but was disappointed there was still no conclusive reason why the engine had failed and was concerned it could happen to owners of the same aircraft.

"There are no answers yet as to why an aircraft that had recently experienced a similar engine failure on the very same engine, when fitted with an after-market turbocharger system with significant manufacturing inconsistencies, was then cleared to fly with a known insecure part on one engine," a family statement said.

The Hertz and Picone families said they still missed the pair deeply and said they remained proud of their accomplishments and examples they set in their lives.

While the CAA has not blamed the medication, Duloxetine, Hertz was on for Generalised Anxiety Disorder or Major Depressive Disorder as the cause of the crash, it has said the American executive would have most likely not been issued a medical certificate and therefore not allowed to fly a plane if it had been declared to either the CAA or the Federal Aviation Administration in the US.

CAA safety investigator Dan Foley said it was one of the most extensive and high-profile investigations carried out by the CAA and involved more than 2000 hours work. The aircraft plunged 56 metres below the surface and a major recovery operation was launched taking 10 days.

The Beechcraft N254F was an American registered aircraft and there was limited oversight in permanently based, privately operated foreign registered aircraft in New Zealand.

A number of safety actions have been raised as a result of the crash including a recommendation that the CAA review the situation of privately owned foreign aircrafts operating in New Zealand.