An emergency alert was issued on Tuesday afternoon for a 93,000-hectare blaze on Myall Creek Road in Bora Ridge, south of Casino, as it increased in intensity in areas of Woombah, Ashby Heights and Old Murrayville Road.

People in Ashby Heights, Ashby and Woombah were urged to seek shelter as the fire approached.

Another emergency alert was issued on Tuesday evening for a bushfire in Ulan, north of Mudgee, but it was later downgraded to "watch and act".

Another longstanding fire at Carrai East near Kempsey burning across more than 118,000 hectares was also at "watch and act" level on Tuesday evening, as was a 138-hectare bush fire just northwest of Arkstone.

A fire in the Currowan State Forest was also at "watch and act" as fire activity increased there and in Shallows Crossing and East Lynne, as was a 76,000-hectare bush fire in the Washpool State Forest.

The Bureau of Meteorology said severe thunderstorms which hit Sydney on Tuesday afternoon were likely to strike the state's northeast later in the night.

"Severe thunderstorms are likely to produce large hailstones and damaging winds in the warning area over the next several hours," the bureau warned after 5pm.

There were 124 bush and grass fires burning across NSW on Tuesday evening with more than 90 uncontained. At least 1200 firefighters remain in the field.

Fire danger ratings on Tuesday were "very high" down the entire NSW coastline. They were expected to be downgraded to "high" on Wednesday with total fire bans in New England and the northern slopes regions.

In Sydney, meanwhile, air quality dipped again on Tuesday sitting between "poor" and "very poor" across the city. It's expected to be "good" on Wednesday.

Building impact assessment teams from the RFS continue to investigate damage in fire grounds across the state.

Some 663 homes have been destroyed in bushfires this season and six people have died.

Australian Associated Press