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.