The CEO of the world's biggest catering company has been named as one of those killed in Sunday's seaplane crash in Sydney.

British resident Richard Cousins, who was CEO of the Compass Group, was among those killed when the plane plunged into a river and rapidly sank.

His two sons, fiance and her 11-year-old daughter were also killed in the crash, alongside pilot Gareth Morgan.

Morgan and his five passengers were returning from a New Year's Eve lunch on the Hawkesbury River when it plunged into the water and sank 13 metres.

Aaron Shaw from Sydney Seaplanes, which owned the aircraft, said the company's thoughts are with the families of the victims.

"We're all just absolutely devastated at his loss and we offer our deepest sympathies and we will support them in any way we can."

Ken Gaunt, also from Sydney Seaplanes, said he's devastated at the loss of Morgan who was his friend, pilot Gareth Morgan.

"It's tough. Pretty shocked, to be honest," he told reporters.

It could still be a number of days before the wreckage can be retrieved from the water.

It is not yet known what caused the crash, but the Australian Transport Safety Bureau is investigating.