Firefighters in southern NSW will seek to take advantage of milder conditions on Saturday as they continue battling a large fire believed to have destroyed 15 homes.

The blaze, which was burning at Carwoola, east of Queanbeyan, on Friday night had already blackened more than 2500 hectares and was proving challenging in hot and windy conditions.

Late in the evening the Rural Fire Service downgraded advice for residents to "watch and act" level and said crews were managing to hold the fire.

"Although conditions in the area are easing and firefighters are slowing the spread of fire in parts, those in the area are to continue to remain vigilant and monitor conditions," the RFS warned.

"A number of properties have been destroyed by the fire. The exact number will not be known until conditions ease."

Assessment teams will move in on Saturday to look at the damage, however the ABC reported 15 homes had been lost.

On Friday night, there were 91 fires burning across the state and 46 were not contained.

The RFS said 100 new fires began after a band of electrical storms swept NSW, lashing greater Sydney.

Residents were encouraged to report any unattended fires and the RFS said aircraft would be up in the morning looking for new areas of concern.

While hot, windy conditions and low humidity tested crews on Friday, Saturday's forecast appeared more forgiving.

Across the Southern Tablelands, the Bureau of Meteorology expected a 70 per cent chance of showers and the chance of a thunderstorm.

Temperatures were expected to reach the mid to high 20s, with light winds becoming northwest to southwesterly at 15 to 25km/h.

The ACT's fire danger was rated as very high by the bureau, while the Southern Ranges was tipped to experience a high danger rating.

© AAP 2017