SYDNEY has smashed previous records, marking its hottest day ever, as regional areas increase bushfire warnings to 'extreme'.

The temperature reached 45.8C at 2.55pm.

The previous record, of 45.3C, was set on January 14, 1939.

The record temperature today far exceeded forecasts, with the Bureau of Meteorology expecting a top of 39C.

But throughout the day temperatures kept rising.

It was already over 40C by 11am. By 12.23pm it had hit 42.7C and by 2.27pm it equalled the previous record of 45.3C.

The Bureau of Meteorology said the temperatures had soared past expected maximums in the city because the sea breeze, which normally cools the coast, was extremely light.

"We basically haven't had much of a seabreeze at all and instead the hot north westerly winds have blown across the city, bringing inland heat," the forecaster said.

Sydney western suburbs also sizzled. Penrith hit 46.5C at 2.16pm and Camden 46.1C at 1.57pm.

A cool change is expected to reach Sydney between 7pm and 9pm tonight, with the relief due in the western suburbs about an hour later than the city.

Sydney's temperature is recorded at Observatory Hill, which has temperature records dating back more than a century.

Read more on The Daily Telegraph.