The fire broke out about 8.30am today on Waimea Valley Rd, northwest of Gore.
Fire and Emergency southern shift manager Lyn Crosson says a 200m-long hedge is on fire and the flames have jumped over to a hay barn.
There are concerns it could spread to a shed and a house nearby.
Four fire engines, four tankers and a command unit are at the scene.
Crosson says conditions are windy and power lines are down but it is not clear whether that is the cause of the fire.