The NSW Rural Fire service has issued widespread total fire bans for Thursday, with hot and windy conditions expected to worsen across the state.

An RFS spokesman on Wednesday evening said there were reports of property damage in the 6000-hectare Three Mile blaze near Wisemans Ferry.

"We've had reports of property damage in the area, we can't confirm exactly what that was," an RFS spokesman said.

An emergency alert was issued for the nearby Gospers Mountain blaze on Wednesday afternoon.

The Gospers Mountain fire has burnt through more than 230,000 hectares in the Wollemi National Park.

The RFS spokesman said there was a chance the two fires could merge as conditions deteriorated on Thursday.

"It's a possibility, we've already seen a large number of fires merging this fire season," he said.

"Conditions are so dry that putting fires out is very challenging at the moment. We are just trying to establish containment lines."

Emergency warnings were in place on Wednesday evening for the Gospers Mountain fire as well as fires near Putty and Paterson.

Strong westerly winds forecast for Thursday could push those three fires further east, putting more properties at threat.

"We are asking people in the vicinity of these fires to have a plan and to know what they will do if that happens," an RFS spokesman said.

The RFS says bushfire smoke is expected to linger in the Sydney basin until the weekend.

"Very heavy smoke is affecting the Sydney area. It is being blown from large fires near Warragamba Dam," the RFS said in a statement.

"Smoke is also affecting areas including the north coast, Hawkesbury, Wollondilly, Queanbeyan and Shoalhaven areas."

Lingering smoke could be "unhealthy for sensitive people, and could cause symptoms, especially in people with heart or lung disease", the state's environment department said.

Air quality levels were again hazardous in parts of Sydney on Wednesday.

Firefighters have forecast an elevated fire risk for Thursday, with severe fire danger ratings stretching along the coastline from the greater Hunter region to the NSW far south coast.

Severe fire danger ratings are also in place for the Monaro region as well as the central and southern ranges.

Total fire bans are in place for all of these areas plus the northern slopes and northwestern regions.

There were more than 130 fires burning across the state on Wednesday evening, with 80 of those fires uncontained. There were more than 2500 firefighters in the field.

Firefighters worked through Tuesday night to protect houses from the 47,000-hectare Currowan blaze north of Batemans Bay on the South Coast.

"There is still a significant threat to life and property at Currowan," an RFS spokesman said on Wednesday evening.

"It's a very large fire spreading to the coast."

Australian Associated Press