It is the find of the century for the River Murray town of Morgan — thousands of telegrams and ledgers dating back 138 years and left long-forgotten under an old postmaster's house.

The historic horde includes mail records and produce ledgers of cargo unloaded at the town's wharf from the paddle boats that plied the River Murray.

Pest controller Tim Owens discovered the hidden hessian bags after cutting a floor trap in the floorboards of the old Morgan postmaster's house during a recent pest inspection.

"I was hoping it was gold, or lots of notes, but it didn't happen like that," Mr Owens said with a laugh.

He ranks it as the most significant find in his nine years working as a pest controller.

"I have found a lot of stuff under houses, tins, bottles, hessian water bags, lights, any rubbish during construction, a lot of it significant to Australia because it's not manufactured anymore," Mr Owens said.

The former postmaster's house in Morgan kept its historic horde secret for more than a century until a pest inspector cut through the floorboards and found bags of well-preserved documents. ( Supplied: Di Hausler )

And while he likes to collect these relics of bygone days, he recognised the value of the find and suggested the homeowner deliver it into the care of Morgan council heritage officer Di Hausler.

"Everyone in my position dreams of a find like this, heritage-wise, this is unbelievable," Ms Hausler said.

Morgan heritage and tourism officer Di Hausler has the task of archiving thousands of documents dating back to 1879. ( Supplied: Di Hausler )

The chance discovery gives an insight into life from 1879, the year after a new railway linked the river trading port with Adelaide, marking the start of a boom period for Morgan.

Ms Hausler now has the job of carefully reading, photographing and archiving every single page.

The postal records and telegrams stop in 1912, the year a new post office was built in Morgan.

"Every letter sent, every mail bag sent to stations at those times, everything was recorded by hand; we've got copies of everything from 1879 to 1912."

Documents in 'pristine condition' for their age

Just how the documents came to be stored or hidden under the building remains unclear, but it seems whoever was responsible sensed they could be valuable for posterity.

"They were in hessian bags, tied with old jute string; some of the hessian bags had salt crystals inside another small hessian bag to keep the moisture out," Ms Hausler said.

She said that those covered by dirt through years of neglect were in the best condition.

"They were in pristine condition considering they were 138 years old."

While a thorough look through all the documents will take many months, already names of River boat captains Hughie King, William Randall and Albert Dunk, and merchant and politician Albert Henry Landseer have been spotted.

"These things are really relevant to Morgan and us being able to piece together our history," Ms Hausler said.

A telegram from Albert Henry Landseer, an English migrant who became a successful river merchant and politician for the Lower Murray district in the late 19th century. ( Supplied: Di Hausler )

Telegrams were the quickest way to send word during the river boat trade era.

"At one point the station master here sent a message to one of the station masters in Kapunda complaining that one of the workers was lazy and inconsiderate," Ms Hausler said of the contents of one telegram.

"'Please write to me' was the message; everything that was important was sent by brief word and then dealt with by a letter afterwards."

Collection to go on public display

There are plans for the collection to be available for viewing by the public once it has been archived.

"The actual town's timeline can be built off these documents," Ms Hausler said.

"It is phenomenal, it is not just people like myself who get excited; it is other people who can see their family's history displayed."