Five Britons including an 11-year-old girl have been killed after a seaplane crashed and sank in a bay near Sydney, Australia, local police confirmed.

The 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.

A sixth person, an Australian pilot, was also killed.

Richard Cousins, 58, and his fiance Emma Bowden, 48, died along with Ms Bowden's 11-year-old daughter Emma Bowden and Mr Cousins' two sons William Cousins, 25, and Edward Cousins, 23. The pilot was named as Gareth Morgan, 44, who worked for Sydney Seaplanes.

An investigation is under way to idetermine the cause of the crash.

The passengers had been on a “wine and dine” sightseeing flight operated by Sydney Seaplanes.

The company said it was “deeply shocked” by the “tragic accident”.

A Foreign Office spokeswoman told The Independent: “Officials from the British consulate are in contact with local authorities in relation to a sea plane accident near Sydney.

“We stand ready to provide consular assistance.”

New South Wales Police Force and the Australian Transport Safety Bureau (ATSB) are investigating the cause of the crash, which happened at about 3.10pm local time (4.10am GMT) on Sunday near to the town of Cowan.

The ATSB said the single-engine plane was reported to have “sunk rapidly” after hitting the water.

Emergency services at the scene of the crash (EPA)

“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.

“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.”

9News reported that the group had flown to a restaurant at Cottage Point and was returning to Rose Bay in the Sydney’s eastern suburbs.

A witness, Myles Baptiste, told the broadcaster 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 added.

“We have spoken to a number of witnesses,” acting police superintendent Michael Gorman said. “It’s too early in the investigation so we don’t know why the plane crashed.”

He added forensics officers would inspect the plane when it had been raised from the riverbed.

Pippa Middleton and her husband arrive at Rose Bay Wharf in Sydney by a sea plane operated Sydney Seaplanes (AFP/Getty) (AFP/Getty Images)

Mr Gorman said were police were in contact with Sydney Seaplanes to identify the victims and inform their families before they were named publicly.

Aaron Shaw, the company’s managing director, said in a statement: “All at Sydney Seaplanes are deeply shocked by this incident and the resulting loss of life.

“We wish to pass on our heartfelt condolences to the family and friends of the passengers and pilot who were tragically killed.

He added: “We do not yet know the cause of the accident.

“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.”

The plane crashed in Jerusalem Bay, near part of the Hawkesbury River (9News)

He added: “Sydney Seaplanes has been operating since 2005, have undertaken thousands of flights in that period and have had an unblemished safety record until now.

“The safety of our passengers and staff is our absolute primary and highest priority.

“Our aircraft are professionally maintained to manufacturer’s specifications and our seaplane pilots are some of the most experienced in the world.”

The company has suspended all operations until further notice, Mr Shaw said.

A string of high-profile passengers have flown on the company’s sightseeing planes, including Pippa Middleton and her husband James Matthews during their honeymoon in May.

Musicians Ed Sheeran and Sam Smith, Microsoft founder Bill Gates, and comedian Jerry Seinfeld have also taken trips with Sydney Seaplanes.

The company offers a four-hour tour, including a three-course lunch at Cottage Point Inn, an award-winning restaurant on the banks of the Hawkesbury River.

The package, which costs 535 Australian dollars (£309) per person, is topped off with a flight over Sydney Harbour Bridge and Opera House, according to the operator's website. Passengers are then returned to the seaplane terminal in Rose Bay.