Between the concrete walls of Darwin's heritage-listed former Reserve Bank building lies a secret room, unseen by human eyes for decades.

It's up a flight of stairs and down a dingy corridor, on a level where employees of the current occupant, Tourism Top End, fear to tread.

There, at the end of the hall, stands an eternally locked armoured door.

Despite the hundreds of keys left behind by bygone bank managers, none have ever managed to throw the door, or this mystery, wide open.

What horrors, traps or treasures lie hidden in this long-forgotten corner of the CBD, nobody really knows.

Will there be piles of gold bullion, left by some forgetful teller before the bank's closure in the 1990s?

Could it be explosive dossiers from the era when the building was used as a headquarters for the United Nations?

Or, as one hardened sceptic suggested, will it hold cobwebbed shelving and a scattering of dead cockroaches?

A map shows the room, called the "voucher room", on the top right hand side. ( Supplied: Top End Tourism )

On Wednesday this week, whatever waits there will be revealed to the world.

Locksmiths have been called in by the treasure hunters at Tourism Top End due to their curiosity finally reaching bursting point.

Building haunted by a ghost called Toastie

Tourism Top End visitor centre manager Julie Lawrance said the room had "never been opened to my knowledge, since the bank closed".

"We've tried every key that we can find, which we have stacks of," she said.

"Obviously being an old bank we have hundreds of keys.

"I'd love to know if there was some money in there, but I don't know."

In addition to having a mystery room, the building is haunted, Tourism Top End's Janine Fiddock said, by a ghost called Toastie, who roams the halls releasing a familiar scent.

"Sometimes you smell toast when you come to work," Ms Fiddock says.

"There was a period of time where the same time every afternoon you would smell this smell of toast, and then it would just disappear.

"It was really quite frightening.

"A few people were actually thinking that it was a presence in this building, to the point that we've often discussed getting it blessed by the Chinese dragon."

A view of Darwin's Reserve Bank, taken from Smith Street, in 1973. ( Source: Cobourg Collection, NT Library )

Building played important role

Director of the NT Government's Heritage Office Michael Wells said he was "quite surprised" when he learned of the building's unopened room a few years ago.

"I did a little bit more research and it turned out it was quite a large room, about 4.5 metres in one direction, and 5.5 metres in the other direction," Mr Wells said.

Being an old bank, it's filled with old teller rooms, "a couple of safes with very large strongroom doors like in the movies, about a foot thick".

Completed in 1967, the building operated as the Territory's Reserve Bank branch for about 30 years, until the deregulation of the banking system.

From 1999 until 2004, the building was occupied by the UN. Mr Wells said "it played a very important role during that era when things were going sort of haywire in East Timor and it was the centre of a great deal of activity" during peacekeeping times.

Tourism Top End moved in about 2007.

Now, in 2019, with the scent of toast still lingering in the corridors, specialist locksmiths are set to put rest to the mystery of the locked room — gold bullion to be found or not.