A seaplane crashed into a river on Sunday, killing six people including four Brits.

An 11-year-old boy and the pilot are also believed to have died in the incident on the Hawkesbury River, near Sydney, ahead of the city's New Year's Eve celebrations.

The sightseeing flight crashed shortly after 3pm local time (4am UK time).

The identities of the victims have not been released.

The aircraft was operated by Sydney Seaplanes, which has been in operation since 1938 and has flown the likes of Ed Sheeran and Bill Gates.

Police said the plane was flying the group from a waterside restaurant in the city to the aircraft's headquarters in Rose Bay, an eastern suburb of the city.

The cause of the crash of not yet known.

Acting Superintendent Michael Gorman said: "Information to date is that at approximately 3.10pm, a sea plane was flying west over the Hawkesbury River.

"For reasons that are not known at this stage, the plane has hit the water and it has subsequently sunk.

"We can confirm the six people on the plane are deceased."

Eyewitness Myles Baptiste told Australian broadcaster 9News: "It made a tight right-hand turn and as it actually turned around, the wings dipped and it nosedived straight into the water."