Three helicopters are fighting a vegetation fire covering about 100 hectares in Hurunui.

Fire and Emergency was alerted to the blaze on Karaka Road, south of the Hurunui River, about 12.50pm on Monday.

The Fenz shift manager said the terrain was too rough for ground operations.

Crews from several North Canterbury brigades were at the scene.

They said there was no indication any homes had caught fire.