A Perth family has made an extraordinary historical discovery after becoming bogged on a West Australian beach.

Tonya Illman was walking across sand dunes just north of Wedge Island, 180 kilometres north of Perth, when she noticed something sticking out of the sand.

"It just looked like a lovely old bottle, so I picked it up thinking it might look good in my bookcase," she said.

But Mrs Illman realised she had likely uncovered something far more special when out fell a damp, rolled up piece of paper tied with string.

The world's oldest-known message in a bottle — a form filled out as part of a German experiment to understand ocean currents. ( Supplied: Kym Illman )

"My son's girlfriend was the one who discovered the note when she went to tip the sand out," she said.

"We took it home and dried it out, and when we opened it we saw it was a printed form, in German, with very faint German handwriting on it."

The message was dated June 12, 1886, and said it had been thrown overboard from the German sailing barque Paula, 950km from the WA coast.

After conducting some of their own research online, the Illman family were convinced they had either made an historically significant discovery or fallen victims to an elaborate hoax.

The form reveals the date the bottle was jettisoned along with the ship's name, home port, co-ordinates and travel route. ( Supplied: Kym Illman )

German experiment in Dutch gin bottle

Between 1864 until 1933, thousands of bottles were thrown overboard from German ships, each containing a form on which the captain would write the date, the ship's coordinates and details about its route.

It was part of an experiment by the German Naval Observatory to better understand global ocean currents.

An artist's drawing of the German sailing barque Paula in 1880. ( Supplied: Edouard Adam/WA Museum )

On the back, the messages asked the finder to write when and where the bottle had been found and return it, either to the German Naval Observatory in Hamburg or the nearest German Consulate.

The Illmans took their find to the Western Australian Museum, where assistant curator of maritime archaeology Ross Anderson conducted a series of investigations.

He determined it was a mid-to-late 19th-century Dutch gin bottle, and the form inside was written on cheaply-made 19th-century paper.

But more needed to be done to shore up the bottle's authenticity, and he contacted colleagues in the Netherlands and Germany for help.

Captain's journal confirms 'extraordinary find'

The colleagues compared handwriting samples from the form and the captain's entries in Paula's meteorological journal.

"Extraordinary finds need extraordinary evidence to support them," Dr Anderson said.

"Incredibly, there was an entry for June 12, 1886, made by the captain, recording a drift bottle having been thrown overboard.

"The date and the coordinates correspond exactly with those on the bottle message.

"The handwriting is identical in terms of cursive style, slant, font, spacing, stroke emphasis, capitalisation and numbering style."

Kym and Tonya Illman with Ross Anderson (centre) from the WA Museum, where the bottle and message are on display. ( Supplied: Kym Illman )

Discovered 132 years after it was tossed overboard, it is the oldest-known message in a bottle in the world.

The second oldest was just over 108 years old.

Kym and Tonya Illman have loaned their find to the WA Museum to display for the next two years.