Police responded to the incident at Palmerston North City Airport after it was reported around 1.30pm.
"The terminal has been evacuated as a precaution while inquiries into the circumstances are made," they said.
Two incoming flights from Christchurch and Auckland have been diverted to Wellington and two flights scheduled to depart to the same cities before 4pm have been cancelled. For flight information or help rebooking, passengers were asked to contact their airline directly.
The airport said earlier, "we will keep you updated with any new information around the terminal opening as it comes to light. We apologise for any inconvenience caused."