Witness describes how his friends risked their lives trying to save British family from sinking seaplane near Sydney

Police and Australian Transport Safety Bureau investigators depart on a police boat to go to the scene of a seaplane crash in the Hawkesbury River near Sydney. Photo / AP

A witness to a tragic seaplane crash in Sydney – in which five Britons and the pilot died – has described watching his friends risk their lives to try to save the passengers and says he wants the grieving families to know: "There was someone there trying to do something."

Will McGovern, who was on a houseboat with three friends when the aircraft plunged into a quiet inlet north of Sydney, said his friends swam over and repeatedly dived underwater – which was "full of fuel" – in a desperate attempt to access the sinking plane.

Emma Bowden and her daughter Heather, 11, were two of the victims of the tragic flight.

"The boys were in the water, diving down, for two to three minutes, trying to help these people inside the plane," he told ABC News.

"The whole time I was freaking out that this fuel was going to spark. The tail was sticking out from the water. This plane was moving fast, it was going down fast — they could have got sucked in… Dead set, they could have died."

Kurt Bratby, one of the men, said he saw the plane's windows when he was underwater but could not see inside.

"It was hard because of the oil, but I could see the windows," he said. "We just couldn't dive down deep enough really to see more."

With the oil fumes burning their eyes, the men tried to tie the tail of the plane to their small dinghy but were unable to lift the aircraft. Ultimately, they gave up as the plane sank about 42 feet to the bottom of the Hawkesbury River.

All six passengers were killed, including Richard Cousins, 58, the head of catering giant Compass, who was travelling with his fiancé Emma Bowden, 48, as well as his sons - William Cousins, 25 and Edward Cousins, 23 - and her daughter Heather Bowden, 11. Gareth Morgan, 44, the pilot, was originally from Canada and had extensive experience flying sea planes.

Sydney Seaplanes plane seen earlier this week. Photo / Tim Wood / Supplied

"The families of these poor people," said McGovern, "they need to know people were there risking their lives trying to help their family members. There was someone there trying to do something."

Investigators have begun piecing together the final moments of the crash but have yet to uncover any fault with the 1964 Haviland DHC-2 Beaver, operated by Sydney Seaplanes. The passengers were returning to Rose Bay on Sydney harbour from lunch on Sunday at the exclusive waterside Cottage Point Inn restaurant, about 20 miles north of the city centre.

Witnesses said the plane took a sharp right turn before "nosediving" into the water.

As reported by The Telegraph, the same model plane crashed in Canada in August 2015 after stalling during a steep turn, causing the deaths of a British family of four and the pilot.

The crash prompted a report by the Transportation Safety Board of Canada in September which recommended that commercial DHC-2 aircraft should be fitted with stall warning devices.

A source told The Telegraph that the plane which crashed in Sydney was not fitted with such a device.

Search and rescue helicopter flies above search boats in Hawkesbury River, New South Wales, where the seaplane went down. Photo / AP

It is understood that few, if any, commercial DHC-2 aircraft in Australia carry the devices.

A spokesman for Sydney Seaplanes said it would not comment on whether a device was fitted while the investigation was underway.

The Australian Transport Safety Bureau, which is conducting the investigation, said it was unable to confirm whether the plane was fitted a device.

Nat Nagy, from the bureau, said the model was typically regarded as reliable.

"It's important not to draw any conclusions that this is a systemic issue," he said.

"We will be looking at the maintenance records of the aircraft, talking to the operator, look at the pilot's background and experience. We'll also be talking to witnesses to try and piece together those final moments before the aircraft impacted the water."

Authorities plan to recover the wreckage by the end of the week – possibly using a crane or airbags - and will examine footage from any mobile phones and cameras on-board.

Cousins, a highly-regarded chief executive, announced in September that he will retire from Compass this year after more than a decade as chief executive. He was due to marry Ms Bowden, a senior editor on OK! Magazine, in July.

They became engaged following the death of his wife to cancer; his sons were due to be best men and her daughter was due to be the bridesmaid.

Cousins, a passionate cricket fan, attended the recent Ashes test in Melbourne with his family and had tickets to attend this week in Sydney.

The Barmy Army, the English cricket team's notoriously rowdy supporters, plan to hold a minute's silence before a light-hearted game today against The Richies, their Australian rivals.