"Police divers are on scene and the six bodies have now been recovered from the wreckage".
Police said they did not immediately know the cause of the crash, nor the identities of the five passengers, but were speaking with several witnesses who were in boats on of the waterways' busiest days of the year.
The Australian Transport Safety Bureau has launched an investigation into the cause of the crash and police said a report would be prepared for the Coroner.
"At around 3pm this afternoon, a DHC-2 Beaver Seaplane, VH-NOO, operated by Sydney Seaplanes was flying in the vicinity of Jerusalem Bay (near Cottage Point)", the bureau said.
The Foreign Office was unable to confirm any details of the crash but said British officials are in contact with authorities in Sydney.
A seaplane makes a low pass over the Sydney Harbour Bridge as climbers enjoy the view from the top arch more than 130 metres above Sydney's Harbour.
Acting Superintendent Michael Gorman said: "Information to date is that at approximately 3.10pm, a sea plane was flying west over the Hawkesbury River".
"We stand ready to provide consular assistance", an unnamed Foreign Office spokeswoman said.
"The plane has hit the water and it has subsequently sunk", Gorman said. Several visiting celebrities have flown on the company's sightseeing planes, including pop stars Ed Sheeran and Sam Smith, tech mogul Bill Gates, comedian Jerry Seinfeld and actor Cuba Gooding Jnr. It originally flew from Australia to Britain, a flight path that required 30 refueling stops along what became famously known as the "Kangaroo" route.