The watchdog's review, which focused mostly on NAB's credit card business, found that the deal would not limit competition in the sector, ACCC said in a statement.
"NAB today is smaller in credit cards than its major bank rivals, and we don't consider adding Citi's card operations to NAB will materially change the level of competition," ACCC Chair Rod Sims said.
NAB, Australia's second-biggest bank, said in August it would buy Citigroup Australia Pty Ltd for about $882.24 million, as the American bank prepares to exit the region in the face of strong challenges to the old credit card business model from buy-now, pay-later companies.
NAB could not be immediately reached for comment.
(Reporting by Sameer Manekar in Bengaluru; Editing by Anil D'Silva)