UPDATED 6.59am: A feared chemical gas leak at Fonterra's factory at Hautapu in Waikato has turned out to be a blown high-pressure air hose.

Seven fire appliances, including a hazardous materials unit from Hamilton, were called to the factory on the outskirts of Cambridge about 3.30am, over concerns of a possible gas leak.

Firefighters in chemical suits and wearing breathing gear entered a service tunnel where they found the blown hose.

Factory maintenance staff were accompanying firefighters into the tunnel to fix the problem.

About 300 people work at the Fonterra factory, which produces milk powder, casein, cheese and other specialist products.

It was not clear if production was affected.