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.

AAP

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 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.

At 8:30am there were 72 bush or grass fires burning across NSW, with 30 not yet contained. Please take five minutes today to discuss your bush fire survival plan. When will you go, what will you take and where will you go if fire threatens? How fire proof is your plan? #NSWRFS pic.twitter.com/ZA1iGhZLyE — NSW RFS (@NSWRFS) December 25, 2019

"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.

READ MORE Christmas proves a slog for NSW firefighters despite cooler weather

"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.

AAP

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 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.

READ MORE How you can help the families of two 'hero' firefighters killed in the line of duty

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.

AAP

"At this time there is no threat to life or property and firefighters are attending this fire," a police warning states.

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.

Police from the Mounted Operations Unit were up in the Adelaide Hills to help out during the #CudleeCreek fires. This officer found a very thirst calf who was appreciative of some water! pic.twitter.com/9aDS9aH7U1 — South Australia Police (@SAPoliceNews) December 26, 2019