NSW is expected to get some reprieve from the heat heading into Christmas - but first the state must endure another "dangerously hot" day across coastal areas.

The Bureau of Meteorology warns that Saturday could be even hotter than Thursday, when two firefighters were killed and several others hospitalised during an exhausting day battling out-of-control bushfires.

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BOM acting NSW manager Jane Golding said temperatures would again rise into the mid-40s, with Penrith expecting a maximum of 47 degrees and coastal areas hitting the mid-to-high 30s.

"That's dangerously hot weather," she told reporters in Sydney on Friday.

A gusty cool change is also expected on Saturday, which will bring some relief from the heat for central and southern NSW but create dangerous conditions on the waterways and challenging fire conditions.

"Those southerlies when they come through (will) be close to gale force," Golding said.

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"The other thing with this system is it is bringing through some swell behind it, so dangerous conditions in the waters on the beach tomorrow as well."

She said there was some respite on the way for NSW as the southerly change moved eastwards towards the ranges.

That should bring cooler temperatures that are forecast to stick around "for quite a few days", with eastern NSW just needing to get through Saturday, Golding said.