Some unusual treasure has been discovered beneath the floorboards of one of Brisbane's oldest buildings.

A long dead cat and a man's boot are thought to be evidence of ancient folk magic.

Workmen tackling rising damp at the National Trust building on William Street in the Brisbane CBD have dug up the objects while carrying out restoration works.

Historians believe the objects were deliberately concealed under the floor as a folk magic ritual to protect the occupants of the building from evil spirits.

Dr Ian Evans has been researching the ritual and its history in Australia.

"This is a story with a very long ancestry," he said.

"What it tells us is that people in Queensland in the 19th century, as in the rest of Australia, believed in and practised folk magic."

Archaeologist Dr Jon Prangnell says the find tells a vivid tale of Queensland's early residents.

"These finds almost certainly relate to a form of protective house magic and I think they are really, really important for the history of Queensland," he said.

He says the boot was beneath a section of floorboards built in the 1890s, and was most likely put there with a specific purpose in mind.

"Underneath the floorboards were clean except for these items, so this shows us that these items were deliberately placed into these cavities," he said.

"These are evidence of the kinds of thinking and the actions of working people who are building Queensland in the 19th century."

Folk magic is thought to have arrived in Australia from England during the convict era.

"There was a belief that you could use boots and shoes as traps for evil spirits," Dr Evans said.

"In 1300 in England there was a famous preacher who was reputed to have conjured the devil into a boot."

Bones of a cat thought to have been placed in the National Trust building as early as 1860. ( ABC News: Courtney Wilson )

The remains of a cat were found in the oldest section of the National Trust building, meaning the dead animal has been lurking underground since at least 1860.

Cats evidently also had a part to play in these ancient practices.

"They would be sent into the other world to deal with spiritual vermin," Dr Evans said.

He says folk magic became the domain of poor English people who felt the Christian churches did not always provide the answers they needed.

"These people were from families that had huge infant mortality rates," he said.

"When they lost a child they often lost faith in God.

"They took the power into their own hands and practiced magic in an effort to protect their families."

To date around 250 concealed objects have been unearthed in buildings across Australia.

The earliest recorded find of a concealed object in Australia occurred next door to the National Trust building in 1913, when a convict shoe was discovered in the attic of the Commissariat Store.

Historians have long believed there are many more hidden treasures awaiting discovery.

It is hoped these latest discoveries will eventually be preserved and put on public display in the places where they were found.