Sydney has sweltered through the city's second hottest night on record and its hottest December night in 148 years, with many forced to cool off at beaches long after dark.

A minimum of 27.1 degrees Celsius (80.8 degrees Fahrenheit) was recorded in Australia's largest city early today, Australian Bureau of Meteorology forecaster Jordan Notara said.

The only hotter overnight minimum temperature recorded in the city of five million people was 27.6C on February 6 2011.

The hottest December minimum nighttime temperature on record had been Christmas Day in 1868, when the temperature dipped to 26.3C.

Records have been kept at Sydney's Observatory Hill weather station since 1859.
"It was mainly due to the hot day yesterday when we didn't have the sea breeze cool things down as significantly as it could have," Mr Notara said.

Sydney was forecast to reach 38C later today, after hitting 37.8C yesterday. Sydney's December average is a maximum of 25.2 Celsius and a minimum of 17.5C.

Beaches, rivers and swimming pools have been crowded with people cooling off.
Electricity supplier Endeavour Energy was able to cope with the increased demand from Sydney air conditioners, company spokesman Peter Payne said.

A cool change was forecast late this afternoon with southerly winds expected to substantially cool the city.

Daytime temperatures peaked above 33C in the south-east capitals of Sydney, Canberra, Adelaide and Melbourne for the first time since 1965.

Like Sydney, the other three cities are expecting cool changes late today.