Kurt Bratby was on a houseboat, getting ready to go wakeboarding, when he heard the plane engine roaring. A moment later, his friend called out that it had crashed.
Mr Bratby, a real estate agent, said he and his friends used two boats to reach the seaplane as it sank rapidly in the waters of Jerusalem Bay, north of Sydney, about 3pm on New Year's Eve.
"We immediately dived down ... we were touching the plane," Mr Bratby said. But fuel obscured the water and within a minute and a half the plane was too deep to reach.
"It could have exploded, it could have done anything," he said.
The Sydney Seaplanes aircraft crash killed British chief executive Richard Cousins, 58, his sons Edward, 23, and William, 25, his fiancee Emma Bowden, 48, her daughter Heather, 11, and pilot Gareth Morgan, 44.
Mr Cousins, the head of the world's largest catering company, Compass, had announced plans to retire in March and the couple had sent out wedding invitations before embarking for Australia.
Nat Nagy, an executive director of the Australian Transport Safety Bureau, said on Tuesday the 1964 DHC-2 Beaver model had displayed no "systemic issues".
Asked whether the model was considered reliable, Mr Nagy said "an aircraft that's been used this long in this many operations, I would say yes".
But he did not know whether it was fitted with a stall warning system, the kind Canada's safety board recommended after the same model crashed and killed six people in Quebec in 2015.
The board said the system should be mandatory as "a last line of defence" but at the time of the September recommendation only four of Canada's 223 commercial DHC-2 planes had one on board.
A spokesman for Sydney Seaplanes said while the company co-operated with authorities "it would not be commenting further on technical issues while the investigation is ongoing".
Three investigators from the ATSB were working to piece together the plane's brief, final flight, looking at factors from pilot history to maintenance to components.
"The aircraft took off in a north-easterly direction, followed by a turn to the north-west, then a subsequent right-hand turn prior to impact," Mr Nagy said.
It settled on the bottom of the creek, 13 metres below the surface, in an "inverted, slightly nose-down attitude". Mr Nagy could not confirm the plane had nose-dived.
Authorities hoped to have recovered the plane by the end of the week by floating it to the surface with airbags, pulling it up with a crane, or some combination of the two.
The managing director of Sydney Seaplanes, Aaron Shaw, said this year people would be surprised at the amount of regulation of seaplanes, which are covered by both maritime and aviation authorities.
The company was "using a lot of technology to stay ahead in what we do and make sure we're operating as best we can", Mr Shaw told Smart Company.
All Sydney Seaplanes' flights remain suspended and the company has offered refunds to customers who did not wish to reschedule.
The suspension comes in peak season for Sydney Seaplanes - Mr Shaw has previously said summer business bought in about 50 per cent of annual revenue.
Mr Shaw co-owns Sydney Seaplanes with business partner Ken Gaunt, who also acts as chief executive of marine safety equipment company Mobilarm.
The safety bureau has appealed for witnesses, especially those with video footage of the plane on New Year's Eve.
Investigators will try to recover any footage taken on the flight from mobile phones or body-cameras before finishing a preliminary report within a month, Mr Nagy said.