From ATSB's website:
***UPDATED at 6.15pm***
The ATSB is investigating the fatal accident involving a single-engine seaplane which crashed into water at Cowan Creek, Hawkesbury River, NSW this afternoon.
Preliminary facts and circumstances as they are believed to be as at 6.00pm on 31 December 2017:
At around 3.00pm this afternoon, a DHC-2 Beaver Seaplane, VH-NOO, operated by Sydney Seaplanes was flying in the vicinity of Jerusalem Bay (near Cottage Point).
It is understood that there was one pilot and five passengers on the aircraft on a return flight to Rose Bay, Sydney Harbour.
The sequence of events leading up to the accident are not yet understood, however following the impact with the water, the aircraft is reported to have sunk rapidly.
The ATSB's Executive Director Transport Safety Nat Nagy and a team of three Transport Safety investigators are travelling to the accident site and will commence their work to determine the causal factors surrounding this tragic accident once the NSW Police activities have concluded.
The ATSB encourages anyone who witnessed the accident to call 1800 020 616 and register their details.
The ATSB will aim to release a preliminary factual report in approximately 30 days.
A final report into the accident may take up to 12 months to complete. However, should a safety issue be identified during the course of the investigation, the ATSB will immediately notify relevant parties.