A grass fire is burning near a state highway outside Christchurch.

The scrub fire was located on the edge of State Highway 1 between Dunsandel and Rolleston, close to Burnham, Tim Reynolds of the Fire Service southern communications centre said.

The blaze, which was reported about 1.30pm, was described as a "reasonable size" at approximately 500m long.

Fire appliances from Rolleston and Dunsandel were at the scene, and tankers from Burnham, Raikaia and Lincoln were also helping to douse the flames.