SYDNEY (BLOOMBERG) - The pilot of a Sydney seaplane that crashed and killed the former head of UK catering firm Compass Group Plc in December 2017 was poisoned by a carbon monoxide leak in the cabin and was unable to fully control the aircraft, investigators said.

The tragedy claimed the lives of Richard Cousins, then chief executive officer of Compass, four members of his family and the pilot.

The plane was on a sightseeing trip north of Sydney when it veered off its flight path, banked sharply and smashed into the Hawkesbury River in a near-vertical position.

The bodies of all six victims were recovered.

The pilot and two of the passengers were found to have "elevated levels" of carbon monoxide, the Australian Transport Safety Bureau said in a statement on Friday (July 3).

The amounts detected "were likely to have adversely affected the pilot's ability to control the aircraft during the flight", the bureau said.

Carbon monoxide is a colourless, odourless and tasteless gas produced by piston engines.

The findings go some way to explaining the New Year's Eve crash, which left investigators baffled.

Witnesses said the DHC-2 Beaver seaplane's engine sounded normal before the disaster, and an initial examination of the wreckage failed to throw up significant clues.

Investigating the source of the fumes, experts found cracks in the plane's exhaust system and three missing bolts under the instrument panel, possibly allowing gases to leak from the engine into the cabin, the ATSB said.

The bureau encouraged piston-engine aircraft owners to install carbon monoxide detectors capable of issuing warnings to pilots.

Cousins and his family had flown north from Sydney's harbour to eat at a restaurant on the banks of the Hawkesbury.

They were minutes into their return leg when the plane went down.