A young boy on a New South Wales beach has found a message in a bottle that was dropped into the Pacific Ocean more than 4,000 kilometres away, by an American college student on a research trip.

Tristan Scilinato, 9, from Kempsey West Public School, said he had been walking on Hungry Head beach south of Coffs Harbour when he found the bottle in the sand.

He said it had been an exciting find.

"We picked it up and we looked inside it and there was the message," he said.

"So we took it home and we opened it with a cork opener because it was really hard to open. Then we got it out of the bottle and we started reading it.

"It was from a girl called Sadie Parsons from California. She dropped it off between Fiji and New Zealand."

Bottle launched on Halloween in 2015

Ms Parsons, who was studying climate change and its effect on island people, dropped the bottle overboard on October 31, 2015.

She wrote, "I threw this bottle over the Robert C Seamans school ship as a drift bottle experiment.

Sadie Parsons launched the bottle off a ship near Fiji on October 31, 2015. ( Supplied: Sadie Parsons )

"We started our journey from American Samoa to Wallis and Futuna then Fiji, and we are now headed to New Zealand.

"If you find this note please contact me. I'm very excited to hear from you."

Ms Parsons ended the note with "Happy Halloween, October 21, 2015."

She said she had been amazed when she got an email saying the bottle had been found.

"We studied components of climate change and sea level rise, different things like that," she said.

"Actually it was the captain's idea for the 23 students to throw bottles overboard.

"So after we were leaving Fiji I said, you know what, I think this is my time.

"I wrote a letter, put it in the bottle, put a cork on top and then we just threw it over.

"I never thought I'd see it again or even know that anybody found it."

Riding in on East Australian Current

Professor of oceanography at the Coffs Harbour-based National Marine Science Centre, Isaac Santos, said it is not surprising the bottle ended up where it did.

"The South Pacific Ocean has something we call a gyre," he said.

The bottle was launched from the Robert C Seamans school training ship. ( Supplied: Sadie Parsons )

"Essentially the water is just going around and around in a counter-clockwise direction.

"So that brings warm waters south from the tropics to along the Australian coastline.

"Part of that circulation pattern is related to the East Australian Current (EAC).

"Near Coffs Harbour the continental shelf is very narrow and the EAC is very close to the land

"That makes it very easy for a little bit of a sea breeze to blow floating debris to our beaches."

Small project highlights big problem

Professor Santos said it was a big adventure for the bottle and Tristan, but there was a more serious side.

"Debris can be a serious problem for marine life. We have a massive pollution problem all over the planet," he said.

"Usually the floating debris accumulates right in the middle of the ocean.

"But occasionally something manages to escape through ocean circulation and reach the shoreline.

"In this case because somebody wrote a message, we know where the bottle came from.

"But most of the time it's very hard to figure out."

Professor Santos said the Coffs Harbour region picked up more marine debris than places further north, such as Queensland's Gold Coast.

"That's because up there the continental shelf is much broader, and you need stronger winds to transport floating debris to the beach," he said.