Richard Cousins, 58, was CEO of FTSE 100 company Compass Group - the world's largest catering company - and died alongside Emma Bowden, 48, and her 11-year-old daughter Heather Bowden.

Mr Cousins's two sons, William Cousins, 25, and Edward Cousins, 23, were also killed in the crash.

Police said the group were on holiday in Australia from the UK, and were on a return flight to Sydney's Rose Bay, close to Sydney Harbour, when the crash happened.

The plane came down off Jerusalem Bay near Cowan, 25 miles north of Sydney centre, at about 3.10pm on Sunday.

The plane was reportedly returning from a restaurant trip when it plummeted into a river.

Superintendent Hutchings added: “At 3.15pm in the afternoon, police and emergency services responded to a seaplane that crashed on the Hawkesbury River near Cowan in an area known as Jerusalem Bay.

“The six passengers on board perished in that crash. As a result of that incident, an investigation on behalf of the Marine Investigative Group and the Australian Transport Safety Bureau into the circumstances of the collision has commenced.

“But before I move on from that, the New South Wales Police Force and the Marine Area Command are in no doubt that the people of New South Wales would like to extend our sincere condolences to the family and friends of those who perished yesterday.

Mr Cousins, who was recently named as one of the world's best-performing CEOs by the Harvard Business Review, was due to stand down as chief executive of Compass in March.

Paul Walsh, the firm's chairman, said: "We are deeply shocked and saddened by this terrible news. The thoughts of everyone at Compass are with Richard's family and friends, and we extend our deepest sympathies to them.

"It has been a great privilege to know Richard personally and to work with him for the last few years.

"Richard was known and respected for his great humanity and a no-nonsense style that transformed Compass into one of Britain's leading companies."

Detective Superintendent Mark Hutchings, head of the New South Wales marine area command, told a press conference the crash "can only be described as a tragic accident".

He added: "These people had come over on holiday to one of the most beautiful parts of the world and for this to happen at a place like this is just tragic."

Emma Bowden was the art editor of OK! Magazine which released a statement saying staff were saddened by the loss of their colleague.

The statement said: "Emma was our Art Editor and had been part of the OK! Magazine team for 15 years, joining in April 2003."

 OK! Magazine editor Kirsty Tyler said: "All the staff at OK! Magazine are deeply shocked and saddened to hear about this terrible tragedy. We would like to extend our heartfelt sympathy to all Emma’s family and friends.

"Emma was a popular and long-standing member of staff at OK! Magazine and she will be greatly missed by us all."

Former editor-in-chief Lisa Byrne also paid warm tribute to her friend and colleague.

She tweeted: “Words cannot express my utter sadness and devastation. 

“Emma Bowden was one of the loveliest people I have ever worked with at OK! Magazine and her daughter Heather was adorable. 

“In total shock when I heard her name on the news. Can't stop crying. My heart goes out to all the family

“I used to call Emma the Grace Kelly of OK! Magazine. Always serene, smiling and never got involved in office politics.”

William Cousins was head of press for Open Britain, which campaigns against a hard Brexit.

The group's chairman Roland Rudd said Mr Cousins was an "extraordinary young man" who would be "missed beyond words".

"All of us at Open Britain are devastated by the tragic loss of Will and his family," he said.

"Will was an extraordinary young man who was passionate about what he did; who took deep pride in his work; and came into the office every day with enthusiasm, energy and determination.

"Will saw his role at Open Britain as more than a job. For him, it was a cause akin to a crusade to stop Britain crashing out of the EU with no deal or a bad deal.

"Will was a great team player who loved the camaraderie of our young team and developed close relationships with all his colleagues. He will be missed beyond words."

The sixth person who died was named as Australian pilot Gareth Morgan, 44, who worked for tour company Sydney Seaplanes.

Chief executive Aaron Shaw described Mr Morgan as a "gentle guy".

He said: "He was a lovely guy. "We are all absolutely devastated at his loss. 

“Ringing his parents today was the one of the worst calls I've had to make in my life.

"We just want to get to the bottom of this as quickly as we can. We are focusing on helping our staff and the relevant authorities."

The tour operator has suspended all flights until further notice.