Australia's second-biggest lender shut all its branches and offices by Wednesday afternoon, citing a "physical security threat", but did not give further details.
Queensland police told The Australian newspaper that a number of NAB branches in the state reported bomb threats.
"The latest advice from State and Federal police is that the threat is not credible and has been deemed a hoax," NAB said in a statement on its website.
It is safe to return to all NAB locations from Thursday morning, the bank added.
A spokeswoman for the Australian Federal Police said NAB was provided with support, but declined to give further details.
(Reporting by Nikhil Kurian Nainan; Editing by Muralikumar Anantharaman and Anil D'Silva)