The Sydney Seaplanes DHC-2 Beaver nosedived into the Hawkesbury River north of Sydney during a sightseeing trip on New Year's Eve, 2017.
The crash claimed the lives of pilot Gareth Morgan and five passengers — high-profile British businessman Richard Cousins, his two adult sons, fiancée Heather Bowden-Page and her 11-year-old daughter.
Air crash investigators previously revealed Mr Morgan had elevated levels of carbon monoxide in his blood at the time the plane hit the water.
NSW Deputy State Coroner Derek Lee on Friday found cracks in the aircraft's exhaust collector ring allowed gases with high carbon monoxide concentrations to pass into the cabin.
"It is most likely that holes in the magneto access panels left open by three missing bolts and gaps around an incorrectly oriented magneto access panel provided the route of entry," Magistrate Lee said.
Exposure to carbon monoxide in the cabin impaired the pilot's ability to operate the aircraft safely, the magistrate added.
The findings follow a 2021 report by the Australian Transport Safety Bureau (ATSB) which put together the aircraft's final movements.
The expected flight path should have seen Mr Morgan taxi away from Cottage Point, take off and turn east to gain altitude over the Hawkesbury River.
Instead, he turned northwest and, flying below the height of the mountainous terrain, entered Jerusalem Bay and made a sharp right-hand turn before the plane dropped nose-down into the water.
Carbon monoxide toxicology testing found he and all passengers had elevated levels of carboxyhemoglobin.
Investigators then found pre-existing cracking in the seaplane's engine collector ring, which could lead to exhaust leakage, and a breach in the plane's firewall, which can allow gases from the engine bay to enter the cabin.
Magistrate Lee recommended that carbon monoxide testing be routinely conducted for all deaths resulting from airplane crashes.
He described the sudden and unexpected deaths of six people in a single event as "truly heartbreaking".
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