Temperatures reached 34.2 degrees in the city just before 1.30pm, making it the hottest day since November 1, the Bureau of Meteorology says.

Somewhat unusually for summer, the central business district should be about as warm as Liverpool and Penrith in the west, fanned by warm winds from inland Australia.

"Those winds are swinging westerly, which makes it hotter on the coast," said James Casey, a meteorologist with Weatherzone.

"Normally you'd have those onshore breezes to keep it cooler."

Humidity levels will also remain high in part because of overnight rain evaporating, he said.

That rain, along with falls earlier this month, means some beaches are worth avoiding as stormwater flows out.

While pollution is unlikely at most beaches around Sydney, Coogee is one place where it can be expected on Wednesday, as is Birdwood Park on the northern beaches, the government's Beachwatch service says.

As usual, southern harbour beaches are harder hit by pollution after rainfall, with Horderns Beach near Bundeena, the Carss Point Baths and most sites on the northern side of Botany Bay worth avoiding on Wednesday.

More showers are expected for Sydney on Thursday and Friday before conditions warm up again on Saturday.

Nationally, the large rain event that caused flooding in parts of inland Australia and soaked south-eastern parts of the country is moving offshore, making way for another build-up of warmth.

"It looks like the heat will start to build from about January 20," Mr Casey said, with Adelaide looking at 35 degrees after a relatively cool patch.

Weatherzone is owned by Fairfax Media, publisher of this website.