In north-west WA, conditions will reach "extreme," with the mercury climbing into the mid to high 40s.

However, Sydney will sail through the week with "great" weather, senior Weatherzone meteorologist Brett Dutschke said.

Sydney's weekly forecast is likely to be "generally near or warmer" than December's average of 25 degrees, with little rain and high humidity, he said.

By contrast, Melbourne will see temperatures of 35 and 38 degrees on Thursday and Friday before dropping to the mid-20s later in the week.

Offering Sydney sanctuary from the heat will be the coastal breezes which keep the city’s temperature cooler.

“To get warm temperatures you need strong northwesterly winds”, said Zhi-Weng Chua, meteorologist at the Bureau of Meteorology.

Out west, though, the mercury will be nudging higher, with four days tipped to reach or exceed 30 degrees in places such as Penrith from Friday.

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