The woman, who has not been identified, arrived in Sydney from Los Angeles and did not have a permit for the firearm, the Australian Border Force (ABF) said.
She could face up to 10 years in jail.
Photos released by the ABF showed an airport scan of the woman's luggage, revealing the firearm inside her bag. A second photo showed the handgun after the bag was opened.
In a statement, an ABF official said that sophisticated detection technology had helped stop a dangerous weapon from entering the country.
