There's no clarification on exactly what the threat to security was at this stage but a NAB spokesman said the bank was working with police.

"Unfortunately we have had to close our branches temporarily due to a physical security threat," the bank posted on Twitter," he said.

"As always the safety and security of our customers and colleagues is our priority.

"We are working closely with police and will provide an update as soon as we can."

The bank has not determined the location or the exact nature of the threat.

The NAB branch on Conadilly Street is the only one in town, but online banking services remain open.