© Kate GeraghtyExercising early at Maroubra Beach on Wednesday.
The mercury is expected to climb to 37 degrees in the city on Thursday and 42 in Penrith, and fall just a couple of degrees shy of that on Friday, according to the Bureau of Meteorology.
All of coastal NSW will endure a heatwave on Thursday, with almost all of it either ranked as severe or extreme. (See bureau chart below). Authorities have also activated the state's heatwave action plan to ensure the public takes care to limit the effects of the heat, such as by staying hydrated and limiting outdoor activity.
The surge of late-December heat means Sydney would notch the city's hottest year in records going back to 1858 "without a doubt", Joel Pippard, a meteorologist with Weatherzone, said. "To not do that, temperatures would have be below zero."
According to Weatherzone, Sydney's maximum temperatures, including Wednesday's top of almost 29 degrees, lifted the average so far this year to 23.76 degrees. That's two degrees above the long run norm, and about a quarter degree higher than the previous hottest year in 2013.
Minimum temperatures will eclipse the previous high set in 2010 by almost half a degree, and are running at an average of just over 15.5 degrees for 2016 with just a couple of days to go, Mr Pippard said.
Comment: Lately, there's been some unusually thick and persistent fog in a number of places, not least northern US, northern China, and northern India.