Sydney's record-breaking weather juggernaut rolls on, with summer confirmed as the city's hottest for both days and nights over records stretching back 157 years.

There's also an increased chance of an El Nino developing later in the year, an event that tends to bring abnormally warm and dry conditions to eastern Australia.

The average of summer days and nights in the Harbour City was 2.8 degrees warmer than usual, beating the summer of 1990-91 as Sydney's warmest, the Bureau of Meteorology said in its seasonal report.

The record warm summer included the hottest ever month, in January, and built on Sydney's hottest year set in 2016. Of the other summer months, December was the city's second-warmest and February equal-second warmest, the bureau said.