Since mid this month Moree has gotten a taste of summer, with temperatures above 35 degrees since November 15.

Moree has already recorded two days above 40 - the hottest was on November 16 when the mercury reached 42.3.

However, we're in for an even hotter week next week as summer officially arrives.

The Bureau of Meteorology says the mercury will start to rise into the high 30s on Friday before hitting 40-plus degrees over the weekend and well into next week.

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Friday is forecast to reach a top of 38 but the temperature only climbs from there. Saturday is expected to get to 39 degrees, while Sunday will hit 42.

Monday and Tuesday are forecast to be a scorching 45 degrees and Wednesday won't be much cooler with a top of 44 expected.

It will also be warm at night with the temperature set to stay above 21 over the next week. Wednesday's minimum is forecast to be 27.

The heatwave is expected to be felt across the state, with temperatures up to 10 to 16 degrees above the average across the state for this time of year with little relief overnight.

"NSW will not be spared with temperatures climbing easily into the mid 40s for vast swathes of the western part of the state and even into the low 40s for some coastal areas," meteorologist Helen Reid said on Wednesday.

"We will see temperatures drop a little bit overnight ... but when you're dealing with a couple of days in a row of well above 40 degrees, it is really pretty much getting into severe heatwave territory."

Hot winds will move across the state from inland Australia, with the worst of it hitting the central parts and the northeast.

The extreme heat and windy conditions would bring a severe fire danger on the weekend particularly in the Northern Rivers region and northwest of the Great Dividing Range.

"It will also be quite windy so we are keeping a close eye on the fire danger which is associated with this warm, heatwave situation," she said.

"It is going to be quite a volatile situation."