Sydney has marked its hottest ever month, with extended warm spells setting records at all metropolitan sites for January mean temperatures.

Last month, Sydney's Observatory Hill notched its warmest January for maximum, mean and minimum temperatures in records going back to 1858, the Bureau of Meteorology said.

The city's mean temperatures averaged 25.6 degrees in January - beating a high mark set in January 1991 by half a degree - and exceeding the long-run norm by 3.3 degrees.

It was also the highest temperature for any month in Sydney, and follows the hottest year set in 2016. The city's average maximum - at 29.6 degrees - shaded the record set in January 1896 by 0.1 degrees, and was also the hottest for any month, Agata Imielska, senior climatologist at the bureau, said.