A member of the public inside the Moray Pl branch dialled 111 about 9.15am when he saw the gun.
At least four police cars were on the scene in moments.
Senior Sergeant Mel Aitken said it turned out the gun was a plastic BB pistol, but police took seriously every report of a firearm being presented.
"These days with plastic guns looking so realistic, it's very difficult even for us to tell the difference, so we have to treat every incident as serious, especially when it's outside a bank."
When he saw police coming, the youth holding the gun deposited it in the Fast Post slot and ran off, she said.
He was picked up in George St by police shortly after.
The youth was charged with unlawfully presenting an imitation firearm and will appear in the Dunedin District Court later this week.
