A Sydney schoolboy has made a stunning discovery in the sand at Sydney’s Balmoral Beach, digging up a $13,376 vintage watch that was at the center of a frantic daylong search.

The watch belonged to a 79-year-old local who lost it after going on his regular morning swim at the popular beach.

The elderly man reportedly removed his 1975 Cartier Santos watch just before 7:30 a.m. on Tuesday and placed it in the pocket of his shorts before taking a dip, according to the Mosman Collective.

An hour later, after returning to his car, his son, Justin, received a call from his distressed father saying the watch was gone.

“Dad was in a real state and very upset,” Justin told the Mosman Collective.

“He parks in the same spot and swims in the same spot every day, but even after retracing his steps, [the watch] couldn’t be found.”

After a frantic search – including literally raking through the sand – for the $13,376 timepiece, the family was connected with local lost item snorkeller Meggan Brummer, who deployed her treasure-hunting son, Josh.

“I got told from my parents that there was a watch lost down (there),” Josh told 7NEWS.

The 10-year-old took to the beach after school, armed with his trusty metal detector and a small shovel.

His mother told the Mosman Collective her son and husband, Simon, “weren’t too hopeful, given the watch was so valuable and had been missing for the entire day”.

But after little more than a minute of searching the sand, Josh started picking up a signal in the sand.

“I dug down about [7 inches] and there was the watch right near the steps that lead onto the beach,” he told the local outlet.

He said he got a “big shock” when he found the watch, but knew it was valuable “because it looked so different from an Apple Watch”.

“As soon as I saw it, I became very excited – I knew I would be able to make the sad man happy again.”

Josh said his elderly father was “overwhelmed” when the 10-year-old returned the watch, which he purchased in France 40 years ago and “is one of his most treasured possessions”.

To thank the boy for his honesty and good work, the family reportedly gave the boy some “pocket money” as a reward.