A fire-generated thunderstorm formed over two South Coast fires on Saturday evening creating "a very dangerous situation" for crews on the ground, the NSW Rural Fire Service said.
On Saturday night, the Currowan fire burning west of Ulladulla was at emergency level and conditions had deteriorated to form the storm above it and the Tianjara fire. Visibility was severely reduced for fire crews on the ground.
The Tianjara fire was downgraded to watch and act as temperatures dropped and spots of rain fell on the South Coast on Saturday evening.
Saturday's extreme and unpredictable conditions caused flames in some areas to reach 60 metres in height.
The northern firefront of the Currowan fire impacted Nerriga about 6pm and residents sheltered in the pub that was being heavily defended, aRural Fire Service spokesman said.
Just after 7pm the spokesman confirmed the fire had moved past the town. NSW Rural Fire Service spokesman Richard Thorek said the heritage museum, township homes and critical infrastructure had been protected but they were unsure of the losses outside of the immediate township.
"It's still smoked in, crews haven't been able to make an assessment as to whether anything has been damaged. The fire did go through there intensely so we are expecting some property losses," Mr Thorek said.
The fire was moving into the Willow Forest Road area and people in that area were urged to remain vigilant and shelter in place. Erratic spot fires were occurring in the vicinity of Tudor Valley Road and Cargills Lane near Braidwood and while conditions had eased residents were told to be aware of any changes to the situation.
Further east, south of the Kings Highway, crews were responding to isolated flare-ups.
"The biggest area of concern is still around Nerriga, and parts of the Nerriga end of Oallen Road where a number of properties are still under threat," the spokesman said.
Residents of Oallen Road near Nerriga were advised to be vigilant through the night and be prepared for the fire to impact. Crews were working in those areas to secure the perimeter of the fire.
On the opposite side of the firefront to the north-east, people in Sussex Inlet, Lake Conjola, Yatte Yattah, Manyaya, Bendalong, Fishermans Paradise and Cunjurong Point were told it was too late to leave and to seek shelter as the fire approached.
The Tianjara fire was burning towards Wandandian and Jerrawangala and people in those areas were also told it was too late to leave.
The Comberton fire near South Nowra remained at watch and act on Saturday night, but the Princes Highway was reopened. Residents near the fire were being warned to be vigilant for spot fires and ember attacks.
On Saturday morning, residents across the state in areas that could potentially be affected by bushfires were urged to leave and spend the day elsewhere.
NSW Rural Fire Service spokesman Richard Thorek said people should "come into town, go to the movies, get some Christmas shopping done, whatever it takes to not be in those areas".
Holiday-makers were asked to reconsider travel for the next few days as fire crews anticipate a "very tough day" on Saturday.
Police urged people travelling in and to NSW to delay all non-essential travel on the roads. Those who needed to travel should monitor conditions, take food and water and carry a fully charged mobile phone.
Surf Life Saving NSW had inflatable rescue boats on standby at various locations up the coast throughout Saturday to help with evacuations if required.
#COMMUNITY // #Thankyou to all the #surflifesavers and clubs assisting with the #NSW fire emergency. We now have members assisting with evacuations at #SussexInlet. Please stay safe 🙏 Photo from #NorthWollongong members who have headed south to help. #nswfirespic.twitter.com/0QDgotGN1K— Surf Life Saving NSW (@slsnsw) December 21, 2019
Sunday's fire danger will be high in the ACT, Far South Coast and Illawarra/Shoalhaven region. After a record setting temperature on Saturday, a top of 28 degrees is forecast for Canberra.
Fire conditions are expected to ease across NSW on Sunday.
On Saturday morning, Shoalhaven Rural Fire Service group captain Paul Gleeson said the outcome of the day would be dependent on wind.
"We've got wind changes today, west to south, we just have to react to what that does and what the fire does," Mr Gleeson said.
"Conditions today will be extreme, we've got low humidities, high temperatures, strong winds, we've got catastrophic fire dangers predicted across the city."
He said a number of fires in the area were out of control, and firefighters were preparing to react to "whatever might come about".
"It's just a bad time.
"We're seeing fire behaviour that's way beyond probably anything we've seen for quite a number of years.
"The dynamics of these fires are that anywhere in the landscape could come under risk at some stage."