All six people onboard a seaplane that crashed into the Hawkesbury River near Sydney have died, NSW police say.

Dive crews are preparing to today refloat the wreckage of the seaplane.

The plane, which went down in Cowan Creek, north of Sydney on Sunday, will need to be brought back to the surface before forensic air-crash investigators can piece together what went wrong.

The sea plane crashed in Sydney's north at 3.15pm Sunday.

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The Daily Mail reports four of the passengers aboard the doomed plane were British, and that one of the passengers was an 11-year-old boy.

However, police said they had not formally identified those on board. Police said the wreckage of the plane, operated by Sydney Seaplanes, was found on the bottom of the creek in about 13m of water, with the bodies inside.

The crash site was about two kilometres north of Cottage Point. Oil and debris was spotted on the surface of the water after the crash.

A Sydney Seaplane spokesperson confirmed it was one of their aircraft that had crashed but couldn't provide any further information.

"Obviously we are very concerned and trying to determine the details at this point. We are working with police at this time," he said.

Dozens of A-list celebrities have taken to the skies on-board Sydney Seaplanes to take in the breathtaking sights of the city and dine in the picturesque waterside restaurants on the Hawkesbury River and Palm Beach.

From royal sister-in-law Pippa Middleton to TV host Jeremy Clarkson, an A-lister's favourite thing to do in Sydney is lunch at top peninsula restaurants Jonah's at Whale Beach, or the Cottage Point Inn, via Sydney Seaplanes.

Pop stars Sam Smith, Ed Sheeran, Ronan Keating and his wife Storm are also among celebs who have flown out on the trip.

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Jerry Seinfeld, Ed Sheeran, Bill Gates and Sam Smith have also had a taste of the premier "fly and dine" experience.

Oscar-winning actor Cuba Gooding Jr, 49, chose the sea planes as his favourite mode of transport while in Sydney early this month to film commercials for Optus.

The Australian Transport Safety Bureau said it will be "investigating the ditching of a single-engine seaplane which crashed at Cowan Creek, Hawkesbury River, NSW this afternoon."

"Three Transport Safety Investigators from Canberra will travel to the site shortly to begin their investigation.

The ATSB encourages anyone who witnessed the accident to call 1800 020 616 and register their details."

A photo of the plane, taken one week prior by Kiwi entrepreneur Tim Wood. Photo / Supplied
A photo of the plane, taken one week prior by Kiwi entrepreneur Tim Wood. Photo / Supplied

Millionaire Kiwi entrepreneur Tim Wood was on the plane a week ago. He said he had a French pilot and flew up for lunch.

"It was pretty bumpy coming into the river valley before we landed for lunch," the ihug co-founder Mexicali shareholder said.

He said Australian music producer Alison Wonderland had also taken the same flight the following day.