A 400-litre gas cylinder is leaking at Mainfreight Transport in Palmerston North, causing an evacuation of the building.
Fire communications shift manager Jan Wills says the top of a gas cylinder had been knocked off, causing the gas to leak out.
One hazardous materials truck and three fire trucks are at the scene, and one person has been taken to hospital for potential gas inhalation.
"They were in the vicinity of the gas so they're being checked," says Ms Wills.
Fire services did not yet know how the top of the cylinder had been knocked off, or what kind of gas it was.
Mainfreight Palmerston North branch manager John Graham says the gas is not hazardous, but staff have evacuated the building so fire services can deal with the situation.