Sydney will sweat through a scorching day on Wednesday, with parts of the city to exceed 40 degrees as firefighters warn of "very high" fire danger for the region.

The Bureau of Meteorology predicts the inner city will reach 36 degrees, while Bondi will have a top of 34, Penrith and Richmond will have a maximum of 42. Campbelltown and Liverpool will reach 38 degrees.

Sydney will sweat through a scorching day on Wednesday. Credit:Louise Kennerley

Angus McLean-Smith, a meteorologist with Weatherzone, said the hottest temperatures are expected in the late morning, driven by north-westerly winds.

A southerly change will then sweep across the city from about midday, with temperatures dropping quickly and rain and thunderstorms possible for the afternoon.