The plane crash that killed 2degrees boss Eric Hertz and his wife was caused by mechanical failure - and not the medication Hertz was taking for a mental illness, a coroner has ruled.

However Coroner Gordon Matenga has recommended changes to the medical certificate application process for pilots after he discovered Hertz was not fit to fly the plane that day because of the medication he was taking.

Hertz was piloting his American registered Beechcraft Baron plane bound for Timaru with his wife Katherine Picone Hertz, 64, on March 30, 2013 when the left engine failed off the coast of Raglan at 11.47am.

A Civil Aviation Authority investigation established Hertz had 38 seconds to recognise that power to the left engine had reduced, causing the plane to "trim" or point its nose up and reduce speed and acceleration.

But before Hertz could take corrective action by switching off auto pilot and gliding toward the Raglan airstrip, the plane went into a spin that could not be recovered from.

CAA safety investigator Dan Foley told the coroner's inquest last February that Hertz would have lost situational awareness because he was flying in cloud, however Foley said Hertz's mental health condition and medication he was taking for it likely exacerbated his disorientation.

Hertz had been diagnosed with general anxiety disorder and major depressive disorder and was taking Duloxetine, a medicine that can cause poor or delayed decision-making, distraction, reduced alertness and incapacitation.

Hertz had failed to disclose his mental illness on medical certificate applications as part of his pilot's licence to both the CAA in 2011 and the Federal Aviation Administration in 2005 in the United States, where Hertz hailed from.

However, Hertz's younger brother Eli Hertz, a former United States Navy pilot, test pilot and instructor pilot, told the inquest his brother was a competent pilot who would have had to overcome a series of difficult issues simultaneously while flying in cloud to keep the plane from stalling and going into a spin.

Eli Hertz said an engineering error during modification of the twin-engine Beechcraft caused the insecure upper deck pressure line in the left engine, that led to the crash that Easter weekend.

The plane had been modified and a turbocharger fitted earlier that month.

The wreckage of the Hertz' Beechcraft Baron was recovered by the Navy in April 2013. Eric and Kathy Hertz died on impact when the plane went down in the Kawhia Harbour near Raglan. Photo / Supplied.

During a test flight following the installation, Hertz experienced a significant drop in engine power and the turbocharger was replaced under the manufacturer's warranty.

But after the new turbocharger was installed a power problem was discovered and inlet ducting was sent back to the manufacturer for inspection where testing found some distortion.

Another test flight two days before the crash was uneventful and N254F was cleared for use after the engineer made minor adjustments to the left engine fuel flow.

Matenga said there was insufficient evidence to make a finding on the cause of the power loss and he did not attribute blame to the engineer who cleared the plane for flight.

In his findings Matenga accepted medical expert testimony from Dr Allen Parmet that Hertz was not affected by Duloxetine or his health problems, but that the cloud the couple were flying in caused Hertz's loss of situational awareness.

There were no cockpit audio recordings of the crash to assess whether Hertz panicked but in the earlier test flight when power was lost an accompanying engineer said the pilot handled the situation "quite well".

Matenga accepted however that had Hertz properly identified his history of depression and anxiety disorder, it was very unlikely he would have been issued with a pilot's licence both in New Zealand and the US.

"My concern is that self-assessment and self-declaration relies solely on the integrity of the applicant," Matenga said.

"Each applicant pilot is aware that his or her answers have the potential to suspend or cancel their ability to fly creating a significant conflict of interest.

"The potential for conflict is increased in circumstances where the applicant is a professional pilot: Tell the truth and have your licence cancelled or suspended, or lie and keep flying. This remains the process for all pilots."

Matenga recommended that CAA and the Ministry of Transport consider amending CAA rules to require an applicant's GP to complete section 20 of the application for a medical certificate, or devise a questionnaire to be completed by the pilot's doctor.

The recommendations, which would require legislative amendment, would be sent to the Transport Accident Investigation Commission and the FAA.

Eli Hertz told the Herald the Hertz family was grateful to Matenga, the CAA and the police for their tireless work investigating the accident that claimed the lives of their much-loved family members.

"Our hope is that other pilots will never find themselves in similarly challenging situation where engine failure at high altitude in the clouds affords no margin of error.

"Kathy and Eric's legacies are honoured by determining and bringing to light the causes of this unfortunate series of events, so that others might avoid the same outcome."

Lochy River investigation

Louisa Patterson and her son James Patterson-Gardner. James died when a Robinson helicopter broke up inflight after a mast bumping event in Queenstown in 2015. (Photo / Supplied)

A coroner has gone further than recommendations made after the Lochy River investigation into the deaths of a helicopter instructor and his student in 2015.

Following findings released today into the deaths of 2degrees boss Eric Hertz and his wife Kathy in a plane crash in 2013, Coroner Gordon Matenga has recommended changes to the way pilots apply for medical certificates as part of their pilot's licence.

Matenga wants GPs to fill out a section of the application to prevent pilots from withholding critical health details that may prevent them obtaining a licence after it was found Hertz did not disclose he was taking medication for a mental illness.

In what became known as the Lochy River investigation by the Transport Accident Investigation Commission, similar concerns were raised when it was discovered the pilot of a Robinson R44 helicopter that crashed near Queenstown in February 2015 had mental health issues the year prior.

It was later found pilot Stephen Anthony Nicholson Combe, 42, who died in the crash in the Lochy River Valley with James Louis Patterson-Gardner, 18, was medically fit to fly and the crash was caused when the rotor blade struck the cabin.

However, the TAIC investigation found there were too many ways for a pilot to circumvent the Civil Aviation Authority process designed to prevent pilots flying if they are not medically fit to.

And there was a low awareness among doctors of their duty to report to CAA if they became aware that a pilot had developed a medical condition that would otherwise render them unfit to fly.

TAIC, with input from CAA, recommended:

• the director of civil aviation improve the mechanisms for informing medical practitioners of the requirements to report to CAA;

• the director review the medical application process to ensure it was more robust in identifying potentially serious health issues with pilots;

• And that a national electronic health record database be able to add a person's occupation and draw attention to health practitioners their obligation to notify the appropriate authority when a patient has a health condition that could pose a threat to public safety.

In his findings released today on the Hertz' deaths, Matenga said his recommendations to legislatively amend the CAA medical certificate processes were appropriate.