The pilot of a seaplane that crashed in the Hawkesbury River in 2017 was likely “adversely affected” by engine fumes in the cabin, the Australian Transport Safety Bureau says.

Gareth Morgan and his five passengers were killed when the Sydney Seaplanes DHC-2 Beaver crashed into the water at Jerusalem Bay on December 31.

The chief executive of catering giant Compass, Richard Cousins, his two adult sons, his fiancee and her 11-year-old daughter were onboard the plane flying to Rose Bay when it went down.

On Friday, the ATSB released an update about its investigation into the incident, saying a toxicology report had found Mr Morgan and two of the passengers had elevated levels of carbon monoxide in their blood.

In a statement, ATSB chief commissioner Greg Hood said: “From that consultation with medical experts, and research into the effects of carbon monoxide on aircraft operations, the ATSB considers the levels of carbon monoxide were likely to have adversely affected the pilot’s ability to control the aircraft.”

Gareth Morgan was an experienced pilot. Photo: ABC

Heavy exposure to carbon monoxide replaces the oxygen in the blood and can lead to tiredness, shortness of breath and confusion.

“Having discounted other potential sources of carbon monoxide exposure, the ATSB considers it likely that the pilot and passengers were exposed to carbon monoxide inside the aircraft cabin,” Mr Hood said.

Investigators also found missing bolts in the firewall, the section that isolates the plane’s engine, which could have allowed the poisonous gas to enter the cabin.

Investigators found no evidence the plane had hit a bird, or that part of it fell off, or that the controls had failed.

Also on Friday, the ATSB published two safety notices designed to prevent and detect carbon monoxide in aircraft cabins, recommending regular inspections for holes and cracks.

The final report is expected to be released later in 2020.