Girl, 11, and family identified as six killed in New Year's Eve seaplane crash on NSW river

An 11-year-old girl is confirmed to be among the dead when a seaplane plunged into the water and rapidly sunk, killing all six aboard, on a New Year's Eve flight over the Hawkesbury River.

The Sydney Seaplanes aircraft was carrying a pilot and five passengers when it crashed at Cowan Creek, Jerusalem Bay, about 30km north of Sydney, close to 3pm on Sunday.

Police, ambulance, Marine Rescue and the Westpac chopper attended the scene, with specialist divers searching the wreckage 13m underwater.

Divers recovered six bodies soon after, with four believed to be British citizens.

Seven News understands 11-year-old Heather Bowden-Page was onboard with her family, on holidays from the UK.

The little girl's mother Emma Bowden, 48, also perished in the crash, along with her fiancé, Richard Cousins, aged 58.

Mr Cousins was the millionaire CEO of catering giant Compass Group.

Also on board were Mr Cousins' sons Edward Cousins 23, and William Cousins, 25, who was a member of anti-Brexit pressure group Open Britain.

Experienced sea pilot Gareth Morgan, 44, was the sixth victim.

It was perfect flying conditions on Sunday afternoon and in the moments before the crash, there were no obvious signs of distress.

Officers said they did not know the cause of the crash, nor the identities of the five passengers, but were speaking with several witnesses who were in boats on one of the waterways' busiest days of the year.

The operator, Sydney Seaplanes, said it was "deeply shocked" by the tragic accident. All operations have been suspended until further notice.

"We wish to pass on our heartfelt condolences to the family and friends of the passengers and pilot who were tragically killed," managing director Aaron Shaw said in a statement.

"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."

Prime Minister Malcolm Turnbull said he lost his father in a small plane crash and called yesterday's incident "a tragic accident".

"Our hearts go out to the families of those whose lives were lost in this accident," Mr Turnbull said.

The British Foreign office confirmed it was in touch with Australian officials amid reports that UK nationals died in the crash.

"Officials from the British consulate are in contact with local authorities in relation to a sea plane accident near Sydney," a spokeswoman said.

"We stand ready to provide consular assistance."

Superintendent Michael Gorman told reporters on the scene on Sunday the passengers had been returning to Sydney after dining at a restaurant on the Hawkesbury, when it crashed into the water, immediately sinking.

The plane was believed to have been travelling from the Cottage Point Inn to Rose Bay, Supt Gorman said.

He said forensic police would inspect the plane to assess when it could be raised from the seabed.

"It's too early in the investigation so we don't know why the plane crashed."

Myles Baptiste said he saw the plane flying towards him about 500 metres away when it hit the water.

"It made a tight right-hand turn and as it actually turned around, the wings dipped and it nosedived straight into the water," he said.

Supt Gorman said police air crash investigators would on Monday comb over the plane for clues about what caused the crash.

The Westpac Life Saver Rescue Helicopter found an oil slick and several items of debris on the surface of Cowan Creek, spokesman Stephen Leahy said.

"There appeared to be a silhouette or outline of a small aircraft but it appeared to be submerged in deep water," he said.

The ATSB said the single-engine DHC-2 Beaver Seaplane "sunk rapidly" after hitting the water.

"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."

Sydney Seaplanes has been operating since 1938, originally flying from Australia to Britain, a journey that required 30 refuelling stops along what became famous as the "Kangaroo" route.

Since 2005, the company has been providing flights above and around some of Sydney's most popular tourist sites including the Opera House, the Harbour Bridge, Pittwater and the Hawkesbury River region.

Anyone with footage of the plane prior to or at the time it crashed into the water is encouraged to contact either the Marine Area Command or Crime Stoppers on 1800 333 000.