Newcastle and Lake Macquarie experienced their hottest day in more than two years but autumn weather is on the way, meteorologist Don White says.
Mr White said temperatures were "well up on average" on Monday, "especially considering it's early March".
"The temperatures are the highest they've been in the Lower Hunter in a couple of years."
However, Mr White added that "this will be the only hot burst for the whole season".
"Temperatures will return to near normal for autumn by the end of the week."
Nobbys hit a high of 36.2 degrees and Cooranbong reached 38.7 degrees on Monday. They were the highest temperatures recorded in those areas since November 2020.
Williamtown also hit a two-year high of 38 degrees, the hottest day there since Australia Day 2021.
The Upper Hunter hit a high of 38.5 degrees at Merriwa, but that was eclipsed on a scorching day in February.
The Bureau of Meteorology issued a warning on Monday for a "low to severe intensity heatwave" in NSW.
It stated that "dry thunderstorms" were possible in eastern NSW on Monday and lightning could "ignite new fires". A strong wind warning was issued for the Hunter coast.
The NSW Rural Fire Service issued an extreme fire danger warning in the Hunter for Monday and Tuesday, with total fire bans in force.
Prolonged wet periods have led to increased vegetation growth, causing concern about high levels of fuel being susceptible to ignition in hot and dry periods.
The Department of Education closed 34 schools on Monday in areas with an elevated bushfire risk across the Central Ranges.
The hot weather will continue, with Newcastle forecast to reach 37 degrees and 33 degrees on Tuesday and Wednesday respectively. It is expected to be slightly warmer in the valley.
Mr White said there was a "lack of really hot days" in summer because there were "a lot of north-easters, rather than northerlies and north-westerlies".
"North-easters, of course, keep the temperatures down - especially near the coast."
He said temperatures across the Hunter in summer, as it emerged from the La Nina pattern, were "slightly above the long-term average, but pretty close to normal".
"This was the first burst of north-westerly or north/north-westerly winds. That means hot air has come across the coast and it will keep the sea breeze out."
Humidity had been "fairly high" because of the north-easters "coming off the ocean".
"The ocean temperatures are relatively warm, in fact they're warmer than normal," he said.
Humidity levels on Monday, though, were "quite low" as the air is "much dryer because it's sourced from the inland".
"Even though it's hotter, the relative humidity is much lower than it's been for the last few weeks."
Mr White said autumn "will kick in" soon.
"It will cool down in NSW towards the end of the week."
Rain during February was below average. Nobbys recorded 70 millimetres that month, compared to the average of 107 millimetres. Williamtown recorded 107 millimetres, compared to the average of 119 millimetres.
The bureau's latest long-range forecast stated that the La Nina phase was "nearing its end".
Autumn was expected to be "neutral", according to the weather outlook that follows the oscillation between the El Nino and La Nina phases.
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