NSW properties could be under threat as increasing numbers of uncontained fires burn across the state.

As temperatures continued to soar on Friday morning, the number of uncontained fires across NSW rose from 14 to 18 in the space of an hour.

As of 12pm (AEDT) there was a total of 99 fires burning across the state.

However, authorities' main concerns continue to be focused on Yass, Cooma and Deans Gap where fires burning up to 10,000 hectares continue to blaze.

At Deans Gap, there are fears the coastal villages of Wandandian and Bendalong could come under threat due to northwesterly winds.

At Yass, where the fire spanned 100 kilometres, firefighters are concerned about possible threats to isolated rural properties.

And at Yarrabin, residents are being urged to monitor conditions, with ember attacks, flame and smoke expected on the eastern side of the fire.

"Those three fires are our main concern for today," Deputy Rural Fire Service Commissioner Rob Rogers told reporters at the RFS Headquarters on Friday.

"They are very large, all either in excess or very close to 10,000 hectares (and) in some cases 100 kilometres of fireline.

"That's an awful lot of fireline for firefighters to monitor and obviously that's going to be troubling over today and particularly tomorrow when it will be even hotter."

About 350,000 hectares have burnt in the fires and 10,000 livestock have perished.

Just one property has been destroyed.

"We've been relatively lucky so far," Dep Comm Rogers said.

"But we're going into a period of warm days for the next few days."

Hot northwesterly winds bringing a warm air mass across the state could exacerbate conditions, Dep Comm Rogers said.

"The winds are not expected to be quite as strong as they were on Tuesday, but given how dry it is and the number of fires we have, we're still quite concerned about the coming days," he said.

Earlier, police and fire authorities condemned arsonists who lit a string of fires at Macquarie Fields in southwestern Sydney on Thursday night.

A number of youths fled the scene and two 16-year-olds were later arrested and charged with breaching bail.

"There's enough fires that start from either accidental or natural causes without dealing with fires that are started by morons who think it's a big joke to light fires in the bush," Dep Comm Rogers said.