When metal detectorist Dale Oldham unearthed a worn coin on a northern New South Wales beach, he didn't think it was anything special.

It was not until the next day he discovered it had come from faraway shores, and originated from Zanzibar, off the coast of East Africa, in the 1800s.

"I had no idea. I thought it was just a rusted Australian $1 coin or 20 cent coin when I first put it in the bag," Mr Oldham said.

"I usually find a lot of can pulls and bottle caps … it was nice to find something a little more interesting."

Dale Oldham says every trip to the beach is like a treasure hunt. (ABC News: Emma Siossian)

Mr Oldham found the coin on December 27 and said it was buried about 20 centimetres down in the sand.

He tucked it away with his other daily discoveries and the next day went online to try to determine what he had found.

"I had a look using AI to search, which narrowed down the possibilities, and from there I could identify some of the markings," he said.

"I could just barely make out some details on there to identify it.

"[It's] an 1881 copper 1 Pysa coin from Zanzibar, I've never found anything like that before."

Dale Oldham says he was just able to make out enough markings to identify the coin. (ABC News: Emma Siossian)

Foreign currencies once common

Coin expert Matthew Thompson, who runs a coin and collectables shop in Melbourne, said the coin was unlikely to hold much monetary value, but was a fascinating find.

He said it was once common for foreign currencies to be used in Australia.

Matt Thompson, holding an 1852 Adelaide Pound, Australia's first gold coin. (Supplied: Matt Thompson)

"If you go back far enough, particularly in the late 1700s and early 1800s, if you look at early colonial New South Wales, to get by, we actually had to rely on currency from all around the world," he said.

 "We didn't have our own mints here in Australia … that didn't come along until the mid-1800s.