An out-of-control grass fire at Pheasants Nest is affecting traffic on the Hume Highway and Picton Road.
Motorists have been advised smoke may cause poor visibility in the area and have been urged to drive to the conditions.
Wollondilly Rural Fire Service Control Centre inspector Greg Freeman said about 15 RFS personnel were working to control the blaze.
He said it was not known how or when the fire started.
He said crews were confident they would extinguish the flames soon.