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 recent milder conditions to strengthen containment lines and more than 1400 were working on Boxing Day to slow the spread of fire.
RFS spokesman Ben Shepherd said they had done great work through Christmas while the conditions remained favourable.
"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 about midday, with 28 yet to be contained.
"Take some time today to prepare your property for the forecast worsening conditions this weekend and early next week," the RFS tweeted.
Bureau of Meteorology duty forecaster Rose Barr said a severe heatwave would 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 told AAP.
"Some areas are forecast to reach extreme heatwave conditions.
"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 1700 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.
"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 deemed "very high" on Thursday for the ACT, greater Hunter, the central and southern ranges, the northern and southern slopes and the NSW north western region.
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.
Australian Associated Press