In those days, Surry Hills was a slum and the children brought before the court were some of Sydney's most impoverished.They needed all the help they could get.

The anonymous gesture of goodwill only came to light last month, during the NSW government's redevelopment of the Albion Street site.

Construction worker Todd Tregent was demolishing internal walls and discovered an old boot in a void behind the fireplace.

"I knew it had historical value but we were working ... so I just put it aside," he said. He later gave it to site manager Gary Rossiter, who alerted archaeologist David Marcus.

"As soon as I heard about where it was from, I realised it was one of these famous concealed shoes," Mr Marcus said.

Since the early 16th century, people have hidden old boots and shoes in chimneys and walls to ward off evil spirits.

The court complex, and the Metropolitan Boys' Shelter it housed, opened in 1911; the boot is evidence that superstitions and faith in folk magic persisted even then.

The boot has been added to the International Register of Concealed Shoes, kept by Northampton Museum in Britain.

The world's foremost authority on historic shoes, Britain's June Swann, identified it as a Balmoral front-lace ankle boot, the kind that "always began as Sunday best".

Mr Marcus said shoes were usually concealed in private homes but in this case "the nature of the building is exactly why it's here".

"It looks like one of the builders hid it," he said. "They knew what they were making. They knew the kids coming through here every day would be the most down on their luck. They did it to look out for the kids."

Historian Dr Ian Evans, an Australian expert on concealed shoes, said footwear was chosen because it took on the shape of the wearer.

"The idea was to decoy evil spirits away from the people," he said. "The feeling was that these spirits were not very perceptive and they'd go and focus on the boot or shoe.

"People at this time were very superstitious.These people lived in fear of death because a simple infection – which today your local GP can cure in no time at all – killed people. There was a great incentive to do whatever it took to preserve your health and safety."

The boot was likely made for a teenager or young adult. The majority of concealed shoes were for children or young people, Dr Evans said, suggesting an intention "to harness the power of the innocent to protect against evil".

NSW Attorney-General Gabrielle Upton visited the court on Friday and said the discovery of the "wonderful" boot reveals the site's historical importance.

The Albion Street court closed in 1983 and is undergoing a $38 million redevelopment.

It will reopen next year, replacing Bidura Children's Court in Glebe, with state-of-the art facilities behind the historic facade.