An investigation is under way into a "suspicious" fire at a Canterbury school in the early hours of this morning.

Fire and Emergency spokeswoman Lyn Crosson said multiple 111 calls came in about 1.45am about a fire at Kaiapoi North School.

When the first two crews arrived they found a building "well alight". Two further crews were requested.

The fire had been extinguished but crews remained on the scene dampening down hotspots.

Crosson said the cause of the fire was not clear but it "seems suspicious".

A police guard was on the scene and a fire investigator would be there this morning to look at possible causes.