The NSW Rural Fire Service is advising residents to prepare their properties ahead of worsening conditions, with soaring temperatures and dry winds set to elevate bushfire danger in parts of the state.
Two firefighters were killed and as many as 100 houses destroyed when bushfires ripped through NSW on Thursday and Saturday last week.
Firefighters have used milder conditions in recent days to strengthen containment lines.
RFS spokesman Ben Shepherd said they had done great work through Christmas while the conditions remained favourable.
A house and car damaged by last Saturday's catastrophic bushfires in the Southern Highlands village of Balmoral. Source: AAP
"The weather has been for once a little bit more in our favour but it's still going to be a potentially difficult day come next week once we start to see that bad fire danger set in," Inspector Shepherd told AAP on Thursday.
There were nearly 70 bush and grass fires burning across the state on Boxing Day, with 30 not yet contained.
"Take some time today to prepare your property for the forecast worsening conditions this weekend and early next week," the NSW Rural Fire Service tweeted.
Bureau of Meteorology duty forecaster Rose Barr said a severe heatwave will start building in southern parts of NSW on Thursday.
"Between Thursday and Saturday we are starting to see severe heatwave conditions in the southern parts of the state, extending over a more significant area of NSW into the weekend and next week," she said.
"Some areas are forecast to reach extreme heatwave conditions.
"With the increasing heat and winds, the fire danger will worsen into the new week, with Monday and Tuesday most likely to be the most significant fire weather days."
More than 1,700 firefighters were in the field on Christmas Day, slogging away to strengthen containment lines before the dangerous weather arrives.
Fire and Rescue NSW commissioner Paul Baxter on Thursday thanked firefighters who worked through Christmas, noting their commitment, dedication, professionalism and selflessness.
A water bombing helicopter helps to contain the Gospers Mountain Fire as it burns towards the village of Bilpin North West of Sydney Source: AAP
"Last week, we had a near miss when a motorist hit one of our firefighters on the incident ground," Mr Baxter said in a post on Facebook.
"I want to remind members of the community that when you are on the road and see emergency vehicle lights flashing, please slow down and give our crews space. It's safer for you and safer for our crews."
Fire danger ratings were rated at "very high" on Thursday for the ACT, the greater Hunter, the central and southern ranges, the northern and southern slopes and northwestern NSW.
There is a high fire danger rating from the far south coast up to Sydney, with a low-moderate danger rating in western parts of the state.
Adelaide Hills bushfire downgraded
Meanwhile, a bushfire blackening Adelaide Hills has been downgraded to the lowest warning level as firefighters work around the clock to extinguish the blaze.
An advice warning has been issued on Boxing Day for Cudlee Creek, Castambul, Chain of Ponds, Charleston, Cudlee Creek, Gumeracha, Harrogate, Inglewood, Kenton Valley, Lobethal, Millbrook, Mount Beevor, Mount Torrens, Paracombe, Tungkillo and Woodside.
"At this time there is no threat to life or property and firefighters are attending this fire," a police warning states.
Exhausted Country Fire Service volunteers catch a short break while battling the Adelaide Hills fire on Friday afternoon. Source: AAP
Thermal imaging equipment had been used to identify hotspots, while large trees burnt and flare-ups occurred on the fire ground.
About 25,000 hectares have already been scorched in the blaze.
One person has died, 84 homes have been destroyed and hundreds of other buildings claimed in the fires.
A heatwave bringing severe to extreme conditions will start moving towards the Australian east coast on Thursday, affecting the northern parts of South Australia.
Adelaide will be spared with a top of 33 degrees Celsius forecast before soaring to 39 degrees Celsius on Friday.
