Engine failure caused the plane crash that killed former 2degrees chief executive Eric Hertz and his wife, but a coroner has questioned how Mr Hertz had a medical certificate after he was diagnosed with mental illness.
A report from coroner Gordon Matenga has recommended changes to how pilots apply for medical certificates after the crash.
The couple were flying a Beechcraft Baron aircraft from Ardmore to Timaru in March 2013, but about 30 minutes into the flight, the aircraft entered into a spin and crashed into the sea, near Kāwhia Harbour.
The coroner's report, released today, questioned how Mr Hertz had a medical certificate as he had been diagnosed with general anxiety disorder and major depressive disorder and was taking Duloxetine, which could impact decision making.
The report found there was not any evidence that suggested Mr Hertz lost situational awareness, instead saying the crash was caused by the left engine failing.
It was not known what caused the engine failure.
However, the report said that Mr Hertz's brother, Eli Hertz, had accepted his brother had lied on his medical certificates, although there was no evidence he was exhibiting any side affects of the medication.
But Mr Matenga said on the day of the crash, Mr Hertz was not "fit to fly".
"I raised concerns with the Civil Aviation Authority in relation to the apparent ease with which Mr Hertz was able to keep his medical condition confidential to his GP and from the knowledge of the Federal Aviation Authority and the Civil Aviation Authority," he said in the report.
Mr Matenga said when Mr Hertz completed the application for a medical certificate, he failed to properly acknowledge he had experience diagnosed depression and anxiety disorders (Section 20 in the application).
He had also incorrectly indicated he not taken any medication in the previous three years for two weeks or more.
"This issue could be easily remedied in my view, by requiring the pilot's ... GP or usual medical practitioner to complete Section 20 of the application for a medical certificate," he said.
"This will ensure that the medical examiner is provided with an accurate medical history."
Mr Matenga made that recommendation to the Civil Aviation Authority and Ministry of Transport.
He said alternatively the Authority and Ministry could devise a questionnaire that would be completed by the applicants GP, and would give the medical examiner an up-to-date medical history of the applicant.