Police are investigating the cause of the crash and were still trying to identify the five passengers and the pilot who were killed in the crash.
Experienced Australian pilot Garath Morgan, 44, died when his aircraft crashed into Jerusalem Bay, along with Britons Richard Cousins, 58, Emma Bowden, 48, Heather Bowden, 11, Edward Cousins, 23, and William Cousins, 25.
The plane was returning the party from a riverside restaurant north of Sydney to Seaplanes' headquarters in the east of the city when it crashed and immediately sank. New South Wales police say divers recovered the bodies of all six victims a few hours later.
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The UK Foreign Office said it was in contact with authorities in Sydney following the incident in Jerusalem Bay, in the Hawkesbury River north of the New South Wales state capital.
"We are dedicating our full resources in assisting the NSW Police, the Australian Transport Safety Board, Civil Aviation Safety Authority and other relevant authorities to understand the cause of the accident".
"We don't know yet what caused it, but it's just a tragedy".
One witness, Myles Baptiste, told Channel Nine the plane was 500 metres from him when it hit the water.
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"It made a tight right-hand turn and as it actually turned around, the wings dipped and it nosedived straight into the water", he added.
Debris and an oil slick have been found on the surface of the water.
He said the crash had affected all the responders who attended the scene.
The ATSB said the single-engine DHC-2 Beaver Seaplane "sunk rapidly" after hitting the water.
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"[We] urge any of the recreational boats that attended the scene shortly after the plane went down to contact either Crime Stoppers or the Marine Area Command or the ATSB".