A seaplane was not following an authorised route when it plunged into a river north of Sydney, killing six people, the flight operator says.
Five members of a British family and a Canadian pilot died in the incident on 31 December. There were no survivors.
A preliminary report, released on Wednesday, did not draw any conclusions about why the plane had crashed.
But Sydney Seaplanes described events moments before the crash as “totally inexplicable”.
The Australian Transport Safety Bureau (ATSB) report said the DHC-2 Beaver had made a steep right turn before diving nose-first into the Hawkesbury River at Jerusalem Bay, about 50km (30 miles) from the city.
“The key question arising from the report is why the plane crashed approximately half way down Jerusalem Bay… the plane simply should not have been where it was,” said Sydney Seaplanes chief executive Aaron Shaw.
Among the victims was Richard Cousins, the chief executive of FTSE 100 catering company Compass Group.
Mr Cousins, 58, died alongside his 48-year-old fiancée, magazine editor Emma Bowden, her 11-year-old daughter Heather and his sons, Edward, 23, and William, 25, and pilot Gareth Morgan, 44.
The pilot had flown the authorised route “hundreds of times before”, Mr Shaw said.
“A turn of this nature at low altitude by a pilot with Gareth’s skills, experience and intimate knowledge of the location is totally inexplicable,” he said.
The ATSB said it was investigating why Mr Morgan had flown in that direction.
“We don’t have a preferred theory at this stage as to why he went off course,” transport safety director Nat Nagy said.
Mr Nagy said investigators were exploring whether the sudden turn was “an attempt to turn around or whether it was a planned turn”.
