At least 12 homes were believed to have been destroyed on the Central Coast on Saturday, as fires tore through the Koolewong area near Woy Woy in an afternoon likened to a tinderbox.
Fires burning at almost all corners of the region, from the Central Coast to the Upper Hunter, to Bulahdelah on the Mid North Coast, triggered multiple emergency alerts through Saturday afternoon as temperatures soared into the high-30s and low-40s in some areas.
Upper Hunter residents near Baerami, Baerami Creek, Widden, Yarrawa and Kerrabee were told it was too late to leave just before 3.30pm as a fire that had blackened more than 10,000 hectares at Milsons Gully was pushed south under gusty winds toward Bylong Valley Way.
"There are homes within the area," RFS spokesman Greg Allan told AAP, but the service was not able to provide more details as conditions were fickle.
Just before 5.30pm, the state's Rural Fire Service advised it remained unsafe to return.
Residents who remained were told to seek shelter if the fire approaches, fill buckets, sinks and bath tubs with water ready to put out spot fires, and be ready to defend against the fire if it approached.
Residents around Sandy Hollow, Giants Creek and Gungal were urged to monitor conditions and be prepared to act.
Evacuation centres were opened at the Merriwa and Denman RSL clubs, as well as Merriwa showground and Denman Pony Club.
Bylong Valley Road remained closed at about 5.20pm, the RFS advised, between Sandy Hollow and Bylong Village, with warnings the Golden Highway could close at any time without notice.
To the south-east, Central Coast residents near Nimbin Road, Glenrock Parade, Lara Street and Nimala Avenue were told to leave immediately and make for Woy Woy as an estimated 100 hectare fire consumed properties and scrub on the banks of Brisbane Water.
Firefighters were quick to respond to the Koolewong blaze, but by the time trucks arrived several homes were already in flames, NSW Rural Fire Service assistant commissioner Leigh Pilkington told the ABC.
After preventing disaster for dozens more nearby homeowners, emergency services were working to ensure the blaze didn't reach the larger hub of Woy Woy.
The federal MP for the region, Gordon Reid, said it was devastating that people had lost their homes in the disaster.
"That's people's lives, that's people's livelihoods," he said.
Residents in the area have been urged to keep abreast of updates, enact their bushfire survival plan and follow emergency services' directions.
Train lines between Gosford and Hornsby are not running, the Transport for NSW advised.
"A very limited number of buses are running between Hornsby, Woy Woy and Gosford. Due to road closures, buses will not stop at Koolewong, Tascot and Point Clare. Extensive delays are expected," the department said in a service alert.
Travellers have been told to delay trips if possible, and allow extra time.
The Newcastle Jets' women's game against the Mariners in Gosford was called off at the last minute amid the extreme heat.
The temperature in Gosford peaked at 40.8 degrees at 3pm, according to the Bureau of Meteorology.
The Hunter and mid-north coast regions were of particular concern for firefighters, with windy conditions expected to fan existing fires and any potential flames.
Several more properties were thought to be affected at Bulahdelah, north of Newcastle, although fire assessors were yet to survey the full extent of the damage.
Just before 4pm, firefighters reported the blaze was moving south towards Booral Road, prompting alerts for residents to evacuate from the area around Crawford River.
One lane in both directions of the Pacific Highway was reopened between Nabiac and Twelve Mile Creek on Saturday afternoon, while Booral Road and numerous roads in the area remained closed.
Hot, west-to-northwesterly winds could cause erratic and variable fire danger, the weather bureau's senior meteorologist Dean Narramore said.
"Dry lightning is also a risk with some of our storms through NSW and that could ignite more fires across the state," he said.
Forecast heatwave conditions baked the region at the weekend as a predicted southerly change late Saturday night or early Sunday morning was expected to bring storms and showers into the end of the weekend.
A severe thunderstorm warning was issued just before 5pm, as forecasters tracked a surface trough triggering severe weather in the hot unstable air mass over the far western parts of the state. The warning area approached the western edge of the Hunter Valley, with forecasters tipping possible severe storms in the region into Sunday afternoon.
Temperatures reached just over 41 degrees at Tocal about 3.30pm, and almost 41 degrees at Gosford, with other areas reaching between 37 and 39 degrees.
Costal areas around Newcastle and afield were relatively cooler, around 32 degrees at 11am at Nobbys and 36 degrees at midday at Norah Head.
Fires to the north of Newcastle sent smoke blanketing over the suburbs, and the Lower Hunter, at the weekend leaving a noticeable haze in the air.
While temperatures were expected to ease from Sunday for most of NSW, the heat is expected to build across northern Australia and WA, returning to much of the country early in the coming week.