A Southland man suffered a medical event while driving a jet boat, losing consciousness before being helped ashore where he would eventually die.

Jet boating NZ immediate past president Paul Mullan said Peter McKenzie was driving the vessel about 1km west of the Wairaurahiti River Mouth when he appeared to suffer a medical event, making the boat lose power as his foot came off the accelerator.

The boat eventually came to a standstill and was overcome by a couple of rogue waves, with the three people onboard trying to stay on as long as possible.

"[The boat] has dropped out from under them."

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McKenzie then regained consciousness and he and another man were helped to shore by the third, younger, passenger.

That man then clambered through 2kms 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, including McKenzie, were whipped by strong winds up to 20 knots and pelted with rain.

Mullan said it's not known what type of medical event McKenzie suffered, and that would be determined by the coroner.

"He was part of the association and obviously he's sadly missed," Mullan said. "Mackcraft are prolific boat builders for jet boats and he's a very, very experienced jetboater. It's just really sad."

It's the second tragic death for his family after McKenzie's brother Morrell died in 2015 when his truck went off the wharf at South Port.

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A statement from the family is expected this afternoon.

The Bluff brothers founded McKenzie Marine and Mackcraft, a jet boat manufacturing company.

Maritime NZ said it was liaising with police.

"Police are continuing to speak with witnesses as part of the ongoing investigation. Maritime NZ has offered assistance should the Police require this and we will be conducting our own report on the incident after the investigation has concluded."

Police said McKenzie's death will be referred to the coroner.