An airport spokeswoman said the terminal had been evacuated about 2pm after what was thought to be a fire alarm activation on the first floor.
Earlier a member of the public, Josephina, wrote on Twitter: "Evacuation at Auckland Airport. Tad bit scary!''
Another said on social media: "Firefighters, fire engines, everyone outside!"
Herald staffer Ella Wilks, who was at the terminal's departure lounge, said alarms had been ringing since about 2pm.
She said things were normal where she was. However, no one had been allowed to come through the security gates heading in to the Duty Free area.
"There are alarms going off downstairs and one one's coming through security,'' she said at 2.45pm.
"They're not letting anyone through and the lights are off.''
Flights did not appear to be affected, she said, and she could see planes arriving and departing from the runway.
Kiwi Brent Melville, now living in Australia, and wife Sue were writing out their departure cards when alarms started sounding throughout the terminal.
He said there was no panic but people started walking back towards the exit when they realised something was wrong.
"There was no smoke, no strange smells or big sounds,'' he said.
"No one knew what was going on. There was no one pointing or on a mega phone... but eventually some people working in the shops started walking out - we all followed.''
He said members of the public stood in the carpark for about 10 minutes as firefighters entered the terminal. But people had been let back in by about 3pm.
Asked whether their flight for the Gold Coast had been delayed because of the evacuation, he said no.
