The crash happened on the Hawkesbury River near the suburb of Cowan.
Police divers have recovered six bodies from the wreckage, which is lying in 13m (43ft) of water.
Unconfirmed reports say the casualties included an 11-year-old boy, the pilot and four British nationals. The single-engine aircraft belonged to sightseeing flight company Sydney Seaplanes.
Investigators do not yet know why the plane crashed.
Acting Superintendent Michael Gorman said the "recovery operation continues" at the site where the single-engine aircraft crashed.