Exposed for the first time in more than half a century is the state of the art (at least it was then) advertising facade of a gentleman's department store that was once a landmark of George Street.

Such works are frequently described as ghost signs but there is nothing too ghostly about the Peapes gent's and boy's outfitters artwork because it has been covered up by the adjacent building over the years and the colours and design remain vibrant and clear.

The building known as Beneficial House is heritage listed, which encompasses the sign and some of the interior. But with the construction next door of a new tower at the Wynyard station upgrade, it won't be too long before, once again, it is lost to view.

Lisa Murray, City Historian, said advertising signs were, by their very nature, ephemeral.

"Part of the charm of the Peapes sign is both its scale and also the vibrancy which make it so attractive," she said.

"If it had been exposed to the sun and rain it would have peeled and disintegrated a long time ago. The uncovering of it is quite magical. This is a once in a lifetime opportunity to see it.

"It would have been a real landmark on George Street. Peapes menswear was an institution that was established in 1866 and by the early 20th century it was known for both tailoring and ready-made clothing for men.

"Before the Sydney Harbour Bridge opened (1932), Wynyard was quite a retailing area so the building itself is a marker of our retailing history down that way."

The Peapes 1922 Christmas Catalogue, describing the new building that opened the following year, states: "It will be a landmark very easy to find. The position is unique, not only because it commands a full view up Hunter Street but also because it is contiguous to the leading hotels, clubs, financial, commercial and shipping offices and it is the centre of the business area most frequented by men and boys."

Peapes ceased trading in 1970 and the building is NOW used as commercial offices. The developer is photographing the sign and the photos will be delivered to the City of Sydney and form part of future archives.

Conserving Historic Signs, NSW Heritage Guidelines, which contains many examples of ghost signs, states: "Historic signs can be an evocative picture of the past, at the top level displaying the cultural climate of a particular era, either economically, politically, socially, scientifically. This can be seen through images of election campaigns, war notices for help in the war effort, graffiti through to advertising products like soap and tobacco."

Author Vanessa Berry, whose book Mirror Sydney to be published next month contains a section on ghost signs, said they were a window into the past enabling people to imagine the city as it once was.

"People might not notice ghost signs that have been there for years [but] really notice this one because it is so bright," she said.

"The '60s was an era in Sydney's history when the prevailing idea was to remove the Victorian city and replace it with a more modern city. I wasn't around at the time but from what I have researched it seems that it would have been an unusual idea that this was, perhaps, worth preserving."

Murray said there were other good examples to be seen around the CBD.

On Sussex Street, where another building has been demolished, is a black and white sign for tentmaker John Walder. A Bushells tea sign on Harrington Street has been restored. In Pitt Street, there are "School of the Arts open till 10pm" and "Rex Simpson fine clothes for men".

She said the signs were a reminder to look up and look around in the city.

"People don't see these things. I always tell people 'Get off your phones and look up.' "