Better weather on Sunday provided an opportunity for critical backburning and containment work ahead of Tuesday, when the mercury is tipped to soar into the 40s in parts of the state.

That work will continue on Monday as about 90 fires burn across NSW, half of which are not contained, and almost 2000 firefighters work in the field.

Authorities are keeping an eye on two big fires in the Singleton area – the Paddock Run and Little L Complex fires – and another Gospers Mountain on Sydney’s northwestern outskirts merging with neighbouring fires.

Very high fire danger is forecast on Monday for several NSW regions including the northwestern region, northern slopes, greater Hunter, central ranges, southern ranges, Illawarra-Shoalhaven and the ACT.

Rural Fire Service commissioner Shane Fitzsimmons said Tuesday would likely bring high temperatures, low humidity and high winds, as well as several thunderstorms with the high risk of lightning strikes.

“There’s going to be a very dry air again as well, humidity is going to be really low, there’s going to be some winds,” Mr Fitzsimmons told the Seven Network.

“Unfortunately tomorrow afternoon with the change, they’re expecting lots of thunderstorm activity and the potential for lots of new lightning and fires.”

He added the thunderstorms were unlikely to produce significant rainfall.

The Bureau of Meteorology has warned much of inland NSW will “swelter” through a heatwave early this week with hot 40-plus-degree days and very warm 20-plus-degree nights, ramping up fire risks.

Dense smoke is also likely to return to the Sydney basin on Monday evening as large fires near Warragamba Dam and the Wollombi National Park intensify.

Mr Fitzsimmons admitted his firefighting crews were beginning to feel the pinch amid an unprecedented NSW bushfire season in which more than 680 homes and six lives 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.

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