The incident occurred at about 3:15 p.m. local time (04:15 GMT) when the plane with five passengers headed from the Cottage Point Inn Restaurant to the Rose Bay suburb of Sydney, the ABC broadcaster reported.
Debris of the plane and an oil slick have been found on the water surface.
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The Australian Transport Safety Bureau (ATSB) has launched an investigation into the incident.
Sea plane crashes into Hawkesbury River in Sydney’s north https://t.co/y3V94AamVi pic.twitter.com/n3yn0FODmN
— AUSTRALIA NEWS (@newsaustralian1) December 31, 2017
Australia sea plane crash: 'Four Brits, 11-year-old boy and pilot dead' after aircraft plunges into Sydney river https://t.co/UC7DXfJCNO
— Manolis Koutselakis (@manolhs) December 31, 2017
.@nswpolice: Forensic police are on their way to the scene after a sea plane crashed into Sydney's Hawkesbury river, killing all six on-board. MORE: https://t.co/92Cy33y9eN pic.twitter.com/islVNKRrif
— Sky News Australia (@SkyNewsAust) December 31, 2017