Visible from a small hole, the 9sq m to 10sq m basement room has a wooden staircase and a wooden floor. The room was discovered when the wall of a known adjacent area of similar size was broken through in 2006.
The room and adjacent area is below the former Bank of New Zealand manager's quarters of the museum, which also includes the kitchen and bedroom. The bank was given to the museum in 1955.
Museum director David Clarke said senior Arrowtown residents recalled a trapdoor behind the counter of the 135-year-old bank building.
Mr Clarke said new display space within the museum was always needed and he knew room was available through the wall.
"Because the wall is rough, I thought the whole thing was foundations underpinning the bank."
However, creating the display was not viable until a $20,000 grant from the Central Lakes Trust was added to $5000 from the museum at the beginning of the year. The museum was quieter in the present shoulder season and so the excavation of the adjacent area began last week.
After the area's neck-high soil and stone rubble had been cleared - ground level in 1875 - Arrowtown engineer Andrew Morris last year assessed the area's structural stability.
Mr Morris shone a torch into the ceiling hole and spotted the wooden staircase and floor in the next room.
Mr Clarke said the museum was considering placing a costumed mannequin of a bank manager, which would be seen loading gold into a safe. A glass window would allow visitors to look through the trapdoor hatch to the room from the ground floor above, as well as from the side, at basement level.
Builders will install consented structural reinforcements, including steel door portals, for the first room, which would become an alcove to view the publicly-closed, but window-paned, safe room.
Mr Clarke said one of the museum's two metal safes could be hoisted in. Interpretation panels would present bank robbery tales from Arrowtown's pioneer past.
Mr Clarke said he expected the museum's new display would be ready by summer.
When asked if spilt gold could be recovered from the safe room, Mr Clarke said: "You never know".
"There's a circa-1940 Denis Glover poem about how `Gold dribbled over the counter, Into the cracks of the floor'.
"Even though the big rush was over when the bank was built in 1875, there were still gold miners operating around the river, the quartz mine in Macetown and farmers utilising the bank.
"By the 1950s, the population had got to such, the bank was no longer viable and they offered it to the museum."
