The Bureau of Meteorology is predicting tops of up to 44 degrees by the weekend in some towns, with little reprieve in the nighttime temperatures of up to 25 degrees.

NSW Rural Fire Service's Allyn Purkiss, deputy incident controller for the area's section 44 fires, said: "The extreme heat coupled with really low humidity is going to be pushing our fire danger ratings a lot higher."

Crews had 27 fires on their radar after lightning strikes mid-last week but the resources to be dealing with only about half of them.

"It doesn't look at this stage like there's going to be a terrible lot of wind, thankfully," he said.

"If we had wind with that, it would be terrible - but as it is, the landscape is so dry that even a little wind under these conditions is making fire behaviour really erratic."

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Bureau climatologist Blair Trewin said temperatures in many areas were forecast to approach or even exceed records for December - or entire years.

"We saw significant heat build over Western Australia over the course of last week, and that heat is now pushing east over the continent, which is going to lead to several days of exceptional heat," Dr Trewin said.

Tamworth is headed for tops of 42 degrees on Friday, and 43 degrees on Saturday and Sunday - after lows overnight of no less than 20 degrees.

The highest temperature recorded at Tamworth airport on a December day was 41.6 degrees on December 30, 1965.

In Narrabri, there could be highs of 44 on Saturday and Sunday, beating the 43.3 degrees recorded at the post office on December 30, 1965.

In Quirindi, the forecast is for 43 on Saturday and 42 on Sunday; the highest temperature on record is 41.8 degrees on December 23, 1990.

Gunnedah's record of 42.4 degrees was on the same day; it is also set for scorching and record-breaking end to the week, heading for tops of 41, 42, 44 and 43 degrees in the four days from Thursday.

The records are set to change even in cooler centres such as Glen Innes, where 36 degrees is predicted for Friday and Sunday, up from the previous highest December temperature of 35.2 on December 19, 1990.

Bureau meteorologist Sarah Scully said there were some steps people could take to stay well.

"For many areas, severe to extreme heatwave conditions will be felt, so it's important people follow the advice of health authorities, stay up to date with the latest forecasts and warnings from the bureau, and remember to check in on any vulnerable family and friends," she said.