It is believed the fire started from a single lightning bolt made ground near a disused airstrip at Gospers Mountain.
It took a weeks but the beast the fire had grown into, arrived at Lithgow on Saturday.
The number of houses lost in Lithgow has yet to be confirmed.
RFS deputy commissioner Rob Rogers said it would be another 24 hours before they had an accurate number of homes destroyed "but I think it is fair to say .. it is around 100".
Areas hit hard by fire include Lithgow and along the Bells Line of Road in the upper Blue Mountains, and the Wollondilly Shire villages of Buxton and Bargo, which were ravaged for the second time in three days.
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