Firefighters have been able to back-burn overnight to slow the progress of a bushfire burning in the Blue Mountains, west of Sydney,

The fire burning between Warrimoo and Blaxland was downgraded to a "watch and act" after a southerly change moved through the area.

Early yesterday afternoon strong winds fanned the blaze, spreading it rapidly from inaccessible bushland and leading to an emergency warning being issued for the area.

The main highway had to be closed, trains temporarily stopped, and children had to be escorted from the Warrimoo public school.

Natalie Sanders from the Rural Fire Service said efforts to contain the fire will continue today.

"Overnight conditions did ease significantly from the southerly that we saw go through last night," Ms Sanders said.

"These easing condition allowed crews to conduct back-burning on that fire to help build containment, so throughout the morning that will continue and favourable weather conditions will actually assist the firefighting effort."

An emergency warning - the highest alert level - was issued on Friday for the bushfire burning between Warrimoo and Blaxland but was downgraded to a watch and act message about 11:45pm (AEDT) on Friday.

The blaze has burned more than 90 hectares of bushland, but no properties have come under direct threat.

More than 250 firefighters were expected to remain in the area to work to contain the fire.

Earlier, authorities warned some Blaxland residents in Baden Place and Bridge Road that it was too late to leave, as crews and six water-bombing aircraft fought the blaze.

The fire had been burning in difficult terrain between two ridges, which meant water-bombing and back-burning was the only option.

Residents are being allowed to return to their homes, but fire authorities are asking them to continue to monitor the situation.

A total fire ban has been declared for Saturday for the Greater Hunter, Central Ranges, Northern Slopes, North-Western, Upper Central West Plains and Lower Central West Plains.

Students escorted from Warrimoo school

Earlier on Friday, firefighters were deployed to Warrimoo Public School due to the fire burning nearby.

New South Wales Fire and Rescue said the students were cared for by staff and crews had been sent to the school as a precaution.

Lara Greenhill, from Warrimoo Public School Parents and Citizens Association, said her son was among a group of 100 pupils who were led away from the school.

"Look, they were pretty calm, I believe...(but) some of them were quite distressed," she said.

Fire trucks lined the grounds of Blaxland Public School. ( 702 ABC Sydney: John Donegan )

"I think they all saw the smoke and they were a bit worried, and then the evacuation alarm did sound, so they all made their way to the hall.

"I haven't actually had that much of a chance to debrief with them as yet, but I believe the evacuation bell went off and they all headed to the hall."

Mark Prince picked up his children from the school after hearing about the emergency.

"I was panicking, I was coming from Wentworth Falls," he said.

Drivers on the Great Western Highway were warned to slow down as smoke billowed over the area. ( Twitter: Andrew Ballard )

"If they went into lockdown they would have been here for six hours and it would have been a long time before they came home, so I had to come down and pick them up,

"They're all fine, we went through the fires in Queensland last year it was just devastating."

Resident Daniel Jennings was working to prepare a friend's house.

"It's pretty full on, I just came from work down at Penrith, just came down right away as soon as I heard there was a fire," he said.

"(We're) just being safe I suppose taking precautions, putting water in the gutters just in case. I'm a bit nervous but it's good to remain calm and get everything done before anything happens."

The fire broke out in terrain that was difficult to access. ( ABC News )

On Friday firefighters battled almost 50 bush and grass fires across NSW.

A warning issued for a grass fire at Coolac, near Gundagai in the state's south, was downgraded late in the afternoon to an advice alert, after an earlier watch-and-act alert.

Fire also threatened two homes at Leeton, north-west of Wagga Wagga, in the afternoon.

Several derelict buildings, a horse stable and a shed were damaged by the fire, which has since been brought under control.

The state sweltered on Friday with some areas experiencing temperatures above 40 degrees Celsius.

In southern NSW, the temperature reached 41.3C in Griffith and 42C at Hay Airport. In the state's west, 43C was recorded at Broken Hill, 43.5 at Cobar and 45.8C at Bourke.

At Penrith, in Sydney's west, the temperature reached 40.9C just after 3:30pm.