"The aircraft is then reported to have entered into an 80 to 90-degree bank angle turn".
The bodies of Mr Cousins, 58, his sons, Will and Edward, aged 25 and 23, Ms Bowden, 48, and her 11-year-old daughter Heather were recovered from the water the same day.
The Cousins family had gone for lunch and taken flight about 3pm to return to Rose Bay, near Sydney Harbour.
Air crash investigators are trying to figure out why a Sydney Seaplanes pilot took an "inexplicable" turn into Jerusalem Bay shortly before his plane crashed killing all six people on board.
"While the exact take-off path from Cottage Point has yet to be established, the aircraft was observed by witnesses to enter Jerusalem Bay".
Canadian-born Gareth Morgan was flying the group - Richard Cousins, chief executive of British catering giant Compass Group, his two sons, his fiance Emma Bowden and her daughter - back to the city from a waterside restaurant when the DHC-2 Beaver aircraft went down.
Witnesses have told investigators the seaplane was flying over Jerusalem Bay at an altitude below the height of the surrounding terrain.
The ATSB report found pilot Gareth Morgan deviated from the usual flight path, which he had flown more than 500 times, before veering sharply at low altitude.
"Several witnesses also reported hearing the aircraft's engine and stated that the sound was constant and appeared normal".
The Australian Transport Safety Bureau has found no evidence the plane broke up or was involved in any other collision, including a birdstrike before the crash.
A full report into the crash is expected in a year.
Their preliminary report provides few clues but confirms the bureau investigated a fatal accident involving the same aircraft in 1996. A periodic inspection of the aircraft was conducted on November 6, with a scheduled engine change carried out.
He was "an extremely experienced float plane pilot who had all the required qualifications and licences", Nagy said.
It rules out a bird strike, contaminated fuel and the plane breaking up in flight as possibilities, and while it does not suggest any likely cause of the crash it does say the flight path is part of the ongoing investigation.
Asked whether mental illness might have been a factor in the crash, Nagy said "we have received no information or facts that there was any concern in that area".
The plane was not required to have any voice or flight data recorder.