A tense two days of firefighting has begun, with changing winds expected to test the containment lines that separate the Hall Road fire from the Illawarra escarpment.
After five days, the fire has claimed more than 14,100 hectares, including two houses and 10 outbuildings in the Southern Highlands and Wollondilly.
It has a circumference of 265 kilometres, and yesterday had come within 12km of the top of the Illawarra escarpment.
NSW RFS crews expect changing winds to make today and tomorrow the toughest days of the containment effort so far.
RFS Illawarra zone manager Superintendent Richard Cotterill said firefighters were bracing for 50kmh winds tomorrow, which could take the fire towards the coast.
"If the containment lines are breached, under the forecast north-westerly winds, fire could run down to the top of the escarpment," Supt Cotterill said.
"Depending on the wind direction and time of day, it's not inconceivable that we could see fire travel down the top of the escarpment, and bring with it exposure to a number of [Illawarra] suburbs."
Authorities upgraded the alert level from Watch and Act to Emergency for residents in Yerrinbool, Balmoral, Yanderra, Pheasants Nest, Wilton, Bargo, Buxton, Hill Top, Alpine/Aylmerton and Couridjah in the Southern Highlands shortly before 3pm yesterday, when an unexpected wind change caused the fire to take an intense run north towards Wilton.
There were confirmed reports of spot fires and embers blowing close to Wilton village, and additional resources including 10 tankers were sent in.
Residents received text message from the RFS, advising them to "seek shelter as the fire arrives".
"The concern was that as the fire progressed north that those embers would fall further north into Wilton township," Supt Cotterill said. "There were a number of isolated properties we had to consider as well."
Picton Road - closed since Thursday - was re-opened yesterday afternoon when the alert level was again downgraded to Watch and Act.