Rural Fire Service units were called to the scene part-way up Jamberoo Mountain around 2.15pm on Tuesday, after calls of a roadside fire.

RFS and Fire Rescue NSW firefighters rushed to the scene to find scrub well alight.

Strong winds foiled firefighters' plans to backburn behind the fire to cut off fuel, while steep terrain and thick scrub also hampered efforts through the afternoon.

"We tried to contain the fire where it is, rather than backburn, due to the temperatures and the winds," said Oak Flats RFS senior deputy captain, Chris Huer.

"It is rugged country to the top of the hill."

Up to six RFS units and four FRNSW trucks responded, with up to 45 firefighters working at the scene throughout the afternoon.

Traffic was blocked along the narrow mountain pass, with drivers forced to detour.

RFS Illawarra zone manager Superintendent Richard Cotterill said investigators had reached the area and would identify the cause of the fire, but Captain Huer believed the fire may have been ignited by a stray cigarette butt.

The road opened to traffic later in the afternoon.

The Illawarra was one of ten regions around the state to be upgraded to a very high fire danger rating, with temperatures into the mid-30s and north-westerly winds gusting up to 50 km/h.

A planned hazard reduction burn at Helensburgh was postponed due to the dangerous conditions