NAB released a statement to its customers on Twitter.

"Unfortunately we have had to close our branches temporarily due to a physical security threat," the statement 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."

Alongside branches, commercial offices which are are co-located with branches have also been closed.

NAB ATMs located at branches are also offline.

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

NAB has branches in the ACT in Gunghalin, Civic, Belconnen, Woden, Tuggeranong and Manuka.