Prime Minister Scott Morrison has paid tribute to NSW firefighters as crews try to contain dozens of bushfires across the state before temperatures soar later this week.

Firefighters had another tough day on Saturday battling blazes including the massive Gospers Mountain mega fire near Sydney’s northwestern outskirts, which has merged with neighbouring fires.

Conditions eased on Sunday, allowing firefighters a chance to do critical back-burning and containment work ahead of Tuesday, when the mercury is tipped to soar into the 40s in parts of the state.

“We’ve got much more benign conditions, particularly a dominant easterly influence which will stretch pretty much right across most of our fire grounds,” NSW Rural Fire Service Commissioner Shane Fitzsimmons told Seven News on Sunday.

“Which means hundreds – as a matter of fact more than 1600 – firefighters are around again today doing really important and critical back burning and containment-line consolidation to try and gain the upper hand before we see those conditions deteriorate into Tuesday.”

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There was a total of 91 fires burning across NSW on Sunday evening, 48 of which were not contained, the RFS said.

Five of these were at a watch and act level.

“Crews have worked hard today locking in containment lines and working to open roads,” the agency posted on Twitter.

The Bureau of Meteorology tweeted a warning on Sunday that much of inland NSW would “swelter” through a heatwave early this week.

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“With hot 40-plus-degree days and very warm 20-plus-degree nights,” it tweeted.

Prime Minister Scott Morrison visited the RFS Control Centre in the Hawkesbury on Sunday for a briefing and update, Mr Fitzsimmons posted on Twitter on Sunday afternoon.

“The PM also delivered a message of thanks to crews in the field, via the radio network.”

Mr Morrison later tweeted: “Our firefighters battling the bushfires are truly amazing.”

Air quality in Coffs Harbour, Bathurst, Tamworth and Gunnedah also reached hazardous levels on Sunday evening.

“Very high” fire danger is in place for Monday for the greater Hunter, Illawarra/Shoalhaven, Monaro alpine, southern ranges, central ranges, northern slopes and north-western fire regions, as well as the ACT.

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AAP.