NSW firefighters are racing to get on top of a number of major bushfires before temperatures soar and lightning potentially sparks new fires, with the premier warning "lethal" conditions await.
Better weather on Sunday provided an opportunity for critical back burning and containment work ahead of Tuesday when the mercury is tipped to soar into the 40s in parts of the state.
RFS commissioner Shane Fitzsimmons says forecast conditions aren't as bad as those experienced earlier this season, but the sheer size and number of fires already burning means the outlook is dire.
"It's not as bad as the catastrophic weather we had weeks ago, but it's certainly going to be another difficult day, particularly given the scale and complexity of these fires and their proximity to so much more built up and populated areas," Mr Fitzsimmons told reporters on Monday.
"We've got fires effectively stretching now from the Queensland border right down to our far South Coast area."
There were 87 fires burning across the state on Monday afternoon with more than 40 uncontained.
There was only one fire at "watch and act" alert level near Tenterfield close to the Queensland border.
Premier Gladys Berejiklian says Tuesday will be "very dangerous" given the combination of very hot weather and windy conditions.
"That combination, as we know, has been lethal in the past few months," she told reporters.
Authorities are working on a so-called mega-fire northwest of Sydney which was formed when the huge Gospers Mountain blaze merged with the neighbouring Little L Complex, Paddock Run, Three Mile and Kerry Ridge fires.
Mr Fitzsimmons expects Tuesday will bring high temperatures, low humidity and high winds - as well as thunderstorms with the high risk of lightning strikes sparking new blazes.
"We saw how deflating it was only a few weeks ago when lightning strikes were starting many new fires just outside of the edge of the containment lines of so many of the fires we had then.
"In one day we went from having 40 fires to something like 165."
The Bureau of Meteorology has warned much of inland NSW will swelter through a heatwave early this week with hot 40C-plus days and very warm 20C-plus nights ramping up fire risks.
There's "very high" fire danger on Monday for several NSW regions including the northwestern region, northern slopes, greater Hunter, central ranges, southern ranges, Illawarra-Shoalhaven and the ACT.
Dense smoke is also likely to return to the Sydney basin on Monday evening as large fires near Warragamba Dam and the mega-fire in the Hawkesbury intensify.
Mr Fitzsimmons admitted his firefighting crews were beginning to feel the pinch amid an unprecedented NSW bushfire season in which six lives and more than 680 homes have been lost.
"They've been flogged now for months now, particularly up in the north ... and (with) the fire activity extending further south, we're literally rotating through thousands of people every day, every week," he said.
The premier urged volunteers to look after themselves.
"Make sure you take the breaks, please know that there are reinforcements there to replace you," she said.
"Please know that if you're feeling the strain, if you need support or assistance, ask for it because we certainly don't want anyone to suffer any further or to experience stress beyond what is already in place."
Nine areas of NSW will be under a total fire ban on Tuesday largely in the east and south of the state.
Australian Associated Press