David Parkinson, whose adventures inspired the film <i>Proof of Life</i>, (pictured) was lost at sea between Niue and Tonga. Photo / Supplied

David Parkinson, whose adventures inspired the film Proof of Life, (pictured) was lost at sea between Niue and Tonga. Photo / Supplied

A New Zealand man was left adrift and helpless on a yacht in the middle of the Pacific Ocean for three days after his skipper mysteriously fell overboard to be lost at sea.

Alexander McDonald, 65, found he was alone on the 40-foot sloop Santana somewhere between Niue and Tonga after he awoke about 6pm on August 20.

British yacht-owner David Parkinson, a former British Royal Marine whose adventures inspired a Hollywood film, is missing, presumed dead.

Tonga police commander Chris Kelley said McDonald was unable to sail the yacht alone but still tried to search for Parkinson.

"He effectively went around in ever decreasing circles for at least three days. He attempted to recover him, but he's been unsuccessful and he [Parkinson] was lost at sea."

McDonald told Kelley he did not know how to operate the communications equipment and was unable to call for help.

After three days adrift, he managed to activate the rescue beacon, which was picked up in the United Kingdom.

British authorities alerted New Zealand that help was needed in the Pacific, and a Tongan Navy rescue boat was sent to save an increasingly worried McDonald.

An aerial search found no sign of the missing skipper.

Parkinson's brother Chris, who lives in the United States, said in an email: "No body and I doubt there'll be one. God knows, I hope his passing was quick and painless - though I doubt it was peaceful."

He believed McDonald had weathered a storm since becoming adrift, having received a message from officials saying: "The engine was broken and the sails were broken."

It was an amazing round-the-world attempt that should never have been possible. The British skipper suffered from Parkinson's disease - yet set off after an experimental operation. Surgeons implanted a pacemaker in Parkinson's chest which sent electrical impulses to electrodes in his brain.

Friend of 20 years Michael Lewis, a photographer based in the United States, had recently sailed with Parkinson. "Having sailed a fair amount I can't imagine anything that is more horrific than being in the ocean and watching the boat leave you behind."

He said Parkinson was "well aware" of his physical limitations. He needed a volunteer crew to help him with sailing - and had given up going in the water.