A heatwave is heading straight for New Zealand after bringing scorching temperatures to Australia.

The heatwave on Saturday in Sydney will drift over the Tasman Sea to the South Island on Sunday, then the North Island on Monday.

Temperatures are predicted to soar as high as 40C through inland New South Wales, and Sydney may hit 36C. Their Government forecaster, the Bureau of Meteorology, reported it would be the first time in history the state has reached 40C in the month of September.

The heatwave should lose around 10C by the time it reaches New Zealand, making for balmy temperatures in the late 20Cs and early 30Cs, WeatherWatch head analyst Philip Duncan said.

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"The air loses a lot of warmth as it leaves hot inland Australia and crosses over the cooler waters of the Tasman Sea. It then warms back up again a bit as it crosses over the ranges of New Zealand."

"If New South Wales is heading to 36 to 40 degrees this weekend it's fair to suggest that the east of the North Island will climb into the mid to late 20Cs.

"I don't expect 30 degrees here, but, I also can't rule it out as the ingredients are there under perfect conditions to climb that high."

WeatherWatch suggested inland Hawke's Bay may reach the mid 20Cs on Monday, but the nor'wester will heat up some areas even higher.

Meanwhile the South Island gets the warm winds earlier, on Sunday. Extra cloud may reduce the heat though. Southland, Otago, Canterbury and Marlborough are all expected to reach the 20C mark, with some making the mid 20Cs if conditions are sunny and windy from the north to northwest during the afternoon.

"Again, we may see some areas go a bit higher if the sun is out and windflow is coming off the hills at the right angle to create additional heat," Duncan said.

New Zealand's complicated geography means not all will feel the heat - those along the Tasman Sea coastline won't be as warm as those in the east will be. And it appeared Wellington would miss out on the warm weather instead copping fairly typical windy spring weather.

Next week cools down faster in the South Island on Tuesday but the North Island stays fairly mild, Duncan predicted.

While sunny warm weather might delight the summer lovers, the early high temperatures are also a warning. Duncan said such high temperatures a full five months out from the hottest time of the year could mean Australia will see record-breaking summer heat and the bushfire season also looks to be severe.