On Saturday, Richmond and Penrith are forecast to reach 41 and 42 degrees respectively before hitting tops of 44 and 45 degrees on Sunday.

While Saturday is a little milder in the inner city - a top of 31 in the city, and 34 degrees at Mascot - it will be hot right across greater Sydney on Sunday.

Weatherzone meteorologist Graeme Brittain said a high of 43 degrees was forecast for both Liverpool and Blacktown on Sunday, and there was potential for the inner city to also hit 40 degrees.

"It could even reach 40 or above at the airport as well," Mr Brittain said.

"Sydney [is forecast for] 38 degrees in the actual CBD, but there's scope for it to be degree or two higher than that depending on the timing of the sea breeze."

There will be little relief early next week, with tops of 33 degrees in the city, 39 degrees in Penrith and 38 degrees in Liverpool with higher humidity than is forecast for the weekend.

"Monday is looking still quite hot, but more humid; temperatures will drop a few degrees on Monday so still in the high 30s for western parts, mid to low 30s further east," Mr Brittain said.

"But you'll still feel a degree or two hotter than that because of humidity, and there is also a chance of showers and storms on Monday."

On Tuesday the temperatures will drop a bit lower still, but the humidity will remain, he said.

Mr Brittain said people should expect more of these hot days throughout January.

"This time of year we see these kinds of temperatures," he said.

"The end of last January we saw this kind of scenario unfold so it's not necessarily unusual."

The record maximum temperature for the inner city in January is 45.8 degrees, recorded on January 18, 2013.

Penrith's January maximum record was also broken that day with a temperature of 46.5 degrees.

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