The NSW Rural Fire Service on Tuesday confirmed 873 homes and 2048 outbuildings have been razed, while another 353 homes have been damaged.

A further 100 homes are also believed to have been lost since Thursday, but they're still being assessed due to limited RFS access.

A total of 73 fires were burning throughout the state on Tuesday night, including the huge Gospers Mountain blaze northwest of Sydney, the Green Wattle Creek fire southwest of the city and the Currowan bushfire on the south coast. Some 28 blazes remain uncontained.

A 'watch-and-act' alert was issued on Tuesday for the 116,000-hectare Kerry Ridge bushfire in the Wollemi National Park, since downgraded to 'advice' level.

Meanwhile, more than 2000 firefighters are making crucial preparations before conditions deteriorate again across NSW this weekend.

RFS commissioner Shane Fitzsimmons said there had been a moderation in conditions, an increase in humidity and moisture in the air which suspended backburning operations from about midnight on Monday.

Rain is forecast for coastal NSW north of Newcastle on Christmas Day, while Sydney will have a 50 per cent chance of rain and a maximum of 26C. Easterly winds will also shift the smoke haze blanketing the city.

"There's very extensive backburning that's going on across a number of these key fire grounds, and particularly some backburning that's very close to properties," Mr Fitzsimmons told Sky News on Tuesday.

Crews were busy establishing these lines on Monday night and would resume backburning as soon as the elevated humidity clears, Mr Fitzsimmons said.

Warmer and windier conditions are expected into the weekend and early next week. However the RFS isn't expecting a repeat of the kind of fire danger levels seen a few days ago, Mr Fitzsimmons said.

"We're really trying to consolidate as much as we can, secure protection as best we can ahead of what's expected to be hotter, drier and, this time, a bit more northerly in the winds," he said.

Those northerlies would have the potential to drive the Gospers Mountain and Grose Valley fires down towards the townships of the Blue Mountains along the Great Western Highway.

"So there's a lot of really difficult, challenging, risky work that continues to be going on and will continue to go on over the coming days, right through the Christmas period," Mr Fitzsimmons said.

Long-term weather forecasts do not predict significant rainfall until January or February.

The federal government, meanwhile, has announced Commonwealth public service volunteers will get at least four weeks paid leave to fight bushfires under a plan to get more "boots on the ground".

Prime Minister Scott Morrison called on big business to follow suit.

"What this will mean, as a big employer, on top of what is done by the defence forces and some of the other agencies, is this will enable them to be able to commit more time in their brigades and relieve particularly those in small and regional towns," Mr Morrison told reporters.

"We're seeking to be a model employer about how we do this."

Mr Morrison also admitted a longer Australian fire season and overlapping periods of major fire danger for states may require policy change, but said "social media is not going to set government policy".

"The longer-term planning and the longer-term policies ... that's what you do in the sober light of day post-event," Mr Morrison said.

Australian Associated Press