A man has been saved after spending 23.5 hours in the water at sea off Whangamatā, encountering a shark, and then using the reflection from his wristwatch to catch the attention of his rescuers.

Whangamatā Police Sergeant Will Hamilton said the man set off on a solo fishing trip on January 2, aboard his 40-foot boat, intending to return the following day.

Near the Alderman Islands he hooked what he suspected was a marlin, before falling overboard about 55 kilometres -- or 30 nautical miles -- off the coast of the North Island.

Unable to catch the idling boat as it meandered further out of reach, the man attempted to swim to the Alderman Islands, but was dragged away by currents.

He endured a cold night in the ocean, too exhausted to keep swimming -- but during his time in the water, a shark came to "have a sniff", before leaving him alone.

At about 2pm on Wednesday, three fishermen on a boat near Mayor Island noticed an unusual reflection on the water.

They discovered the fisherman in the sea, who had got their attention using the reflection of the sun on his watch.

The trio plucked him from the water, immediately alerted Police and set course for Whangamatā Marina.

Hamilton said it was an "absolute miracle" the fisherman was alive.

The man was hypothermic and exhausted, and received treatment from a St John ambulance crew on-shore.

The man said he wanted to thank Mike, Tyler and James for rescuing him, along with all emergency services involved.

The man's boat was still missing, and boaties are asked to report any sightings of empty powered vessels.

This story was originally published by RNZ