Temperatures across the Hunter are expected to reach the mid 40s later this week with predictions the heatwave could set new December records.
Newcastle is tipped to reach 36 on Thursday and 40 degrees on Saturday while Cessnock is tipped to hit 43 on Thursday and 45 on Saturday. Scone is forecast to reach 42 degrees on Thursday and 45 on Saturday.
Bureau of Meteorology meteorologist Adam Morgan said it was likely numerous December records could be broken.
"The existing record for the highest maximum temperature averaged over Australia for a day is 40.3C. That was set back in January 2013," he said.
"Current indications are that we'll be at least a degree above that on Wednesday and Thursday, so a really extreme event from a nationwide perspective."
The hottest temperature for Newcastle was 42.5 recorded on January 18, 2013, Cessnock reached 46.8 on February 11, 2017 and Scone reached 46.5 on February 12, 2017.
The state's firefighters are also bracing for the "worst possible circumstances" as the heatwave combines with low humidity and strong winds.
Of the 115 blazes burning across the state on Tuesday 59 burning out of control.
"We will do whatever we can to prepare communities. People should be under no illusion, we won't contain the fires by the time the weather deteriorates later this week," Rural Fire Service deputy commissioner Rob Rogers told the Today Show on Tuesday.
"I think Wednesday, Thursday are looking like problem days. Saturday is looking potentially even worse.
Health authorities have urged people to check in on any vulnerable family and friends.
