Workers redeveloping the Sarjeant Art Gallery found a cannonball while excavating the site yesterday evening.
Whanganui Mayor Hamish McDouall told Morning Report they were concerned it could be explosive and had to call in the bomb squad.
"We've been finding quite a lot of things there over the last few weeks as the excavation goes on, lots of marbles and toy cards, because there was a school there, but going a layer deeper suddenly there's a metal cannonball, and the site was evacuated."
It was now being examined by WorkSafe and the bomb squad, but they were still unsure about which war or year it was from, McDouall said.
"That's the key issue, I mean there was shrapnel cannonballs produced well before 1840, but I've been [learning about] cannonballs pretty swiftly which is something normally a mayor doesn't do.
"WorkSafe are going to be X-raying the cannonball and if it's explosive then they'll blow it up."
He said they would love to keep it if it turned out to be non-explosive, along with the other "treasures" they'd found at the site.
"It could be just a shot, which is just a solid metal ball, but if it's hollow it could have an explosive charge in it."
But it was not the first time the city has encountered dangerous finds, the mayor said, because the site around the gallery had known historic battlegrounds.
"In fact, a shrapnel bomb around by the time of the Rutland Stockade which used to be situated on Pukanamu, which is where the art gallery is, it could well be one of them as well."
"It's a very historical area, an old sand dune essentially, and the Rutland Stockade was just up from the site, and the area there was known as 'the rookery' for many years after [troops left] from the late 19th century, because many squatters lived up there.
"So this won't be the last major find but it's certainly the most, how shall I say, explosive."