The city's top temperature skipped past the 30-degree barrier on December 17 and Orange's maximum day-time temperature has almost exceeded that mark every day since, with December 23 maxing out at 29.8 degees.

The last few days of 2019 are all tipped to push past the 30-degree mark, though adding to an already record-breaking month for the city.

The average top temperature for Orange throughout December is 24.7 degrees - the city has been above that mark since December 5 - and the previous top before 2019 was 34.7 in 2009.

Orange has already passed 34.7 degrees four times this year and is predicted to for a fifth time with new year's eve's forecast of 35 degrees.

The average temperature this month has been 28.6 degrees Celcius, 3.9 degrees above average and a record mean for the city.

Bureau of Meteorology duty forecaster Rose Barr said a severe heatwave would start building in southern parts of NSW at the end of the week.

"Between Thursday and Saturday we are starting to see severe heatwave conditions in the southern parts of the state, extending over a more significant area of NSW into the weekend and next week," she said.

"Some areas are forecast to reach extreme heatwave conditions.

"The fire danger will worsen into the new week, with Monday and Tuesday most likely to be the most significant fire weather days."

Orange has had three days of rain in December, recording 15mm, well below the mean mark of 91.5mm.

Meanwhile, on the back of the heatwave-like conditions to end 2019 the NSW Rural Fire Service is advising residents to prepare their properties ahead of worsening conditions, with soaring temperatures and dry winds set to elevate bushfire danger in parts of the state.