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Peter McKenzie and his late brother Morrell founded McKenzie Marine and Mackcraft, a jet-boat manufacturing company.
Jet Boating NZ immediate past president Paul Mullan said Peter McKenzie was driving the vessel about 1km west of the Wairaurahiri river mouth on Saturday, when he appeared to suffer a medical event, resulting in his foot coming off the accelerator.
The boat was then hit by a couple of rogue waves, Mr Mullan said.
The three people aboard tried to stay with the boat as long as possible.
''[The boat] has dropped out from under them,'' Mr Mullan said.
Mr McKenzie regained consciousness and he and another man were helped to shore by the third and younger passenger.
That man then clambered through 2km of rugged terrain on his way to Waitutu Lodge.
The remote lodge had no cellphone reception and the man had to radio for help.
Emergency services arrived about four hours later.
As the man ran for help, the two remaining men were whipped by strong winds of up to 20 knots and pelted with rain.
Mr Mullan said it was not known what type of medical event Mr McKenzie suffered.
''He was part of the association and obviously he's sadly missed,'' Mr Mullan said.
''Mackcraft are prolific boat builders for jet-boats and he's a very, very experienced jet-boater. It's just really sad.''
In a statement last night, the McKenzie family said his death was a huge loss.
They said the experienced jet-boater was ''extremely safety conscious and knew the area well''.
Son Daniel McKenzie said from witness accounts it appeared his father had a medical event.
''We will have to wait for the postmortem results to be sure of what happened, but Dad knew that part of the coast well and the two passengers say they felt safe.
''Dad respected the sea and was an expert boatie.
''We are devastated and ask for privacy so we can come to terms with this awful tragedy.''
Morrell McKenzie died in 2015, when his truck went off the wharf at South Port.
Maritime New Zealand said it was liaising with police over Saturday's death.
Police said the death would be referred to the coroner.
-Additionally reported by the New Zealand Herald