The owner of a seaplane that crashed near Sydney during a New Year's Eve joy flight, killing the Canadian pilot and his five British passengers, says that flight path was not authorized.

The Australian Transport Safety Bureau on Wednesday released a preliminary report on its investigation into the cause of the de Havilland Canada DHC-2 Beaver crash into the Hawkesbury River.

Sydney Seaplanes chief executive Aaron Shaw said in response to the report that its key question was why the plane was flying in a bay surrounded by steep terrain that had no exit.

Shaw said in a statement Sydney Seaplanes had not authorized the route and "the plane simply should not have been where it was."

The report rules out potential causes but does not suggest any likely cause.