P Kuiper tweeted: "#firealarm at #christchurch #airport everyone evacuated to the main entrance."
Odette Coates posted: "Just been evacuated from the airport and the fire service are in the building."
However, the emergency was short-lived, as firefighters quickly discovered it was a false alarm.
A spokeswoman for the Fire Service southern communications centre said the fire alarm was activated "possibly by bug spray, but that's not confirmed".
On Twitter there was speculation that dust had caused the alarm to go off.
A spokeswoman for Christchurch Airport confirmed the false alarm, and said "nobody was ever in any danger".
"The fire alarm was activated, and we evacuated the terminal at around 4pm, for about 20 to 25 minutes," she said.
"Everybody is back in again now, and we're running as normal."
She said it was not yet known what had sparked the alarm.