Firefighters have contained a large grass fire south of Canberra this afternoon.

At about 3pm, NSW Rural Fire Service responded to reports of a fast-moving grass fire on the Monaro Highway at Colinton.

Upon arrival firefighter found a large grass fire with properties and assets at risk.

The fire had run up into bushland in rough terrain making containment and control difficult.

The fire was brought under control just after 6pm this evening after burning through 105 hectares.

During the height of the fire 30 firefighters in 15 trucks attended the scene with the support of one water bombing helicopter and one grader.

The cause of the fire is believed to be from an escaped pile burn, and there were no reports of serious injury.