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.

Summer was marked by "both the lack of cold during the season and several significant heatwaves", said Acacia Pepler, a bureau climatologist, noting the pattern fitted in with climate change.

"The number of warm days are increasing across the country and the globe," she said.

Among the many highlights of the season were the most days on record above 35 degrees, with 11 such days compared with an average of two in a typical summer.

Almost a whole month endured days above 30 degrees, with 26 of them during the summer, or more than triple the nine usually recorded.

While Observatory Hill missed out on a 40-degree day during the summer, inland regions baked with Richmond posting a record of 11 summer days of at least 40 degrees. The city's hottest day was on January 31, with 39.4 degrees reached.

NSW and Australia

Statewide, it was also a record summer for both maximum and mean temperatures in NSW, with the heatwave that ran to February 11 among the most remarkable such events in Australian history, the bureau said in a recent report.

Across the state, area-averaged temperatures were 2.57 degrees warmer than normal, eclipsing the previous record set in 2005-06 by 0.13 degrees.

On February 11, the state's average maximum was 44 degrees, beating the previous record for the month prior to this year by a full two degrees. Moree's run of 54 days of at least 35 degrees also set a fresh record for NSW. 

Nationally, it was Australia's fifth-hottest summer on record for minimum temperatures at 0.82 degrees above the 1961-90 average. Mean and maximum temperatures were also warmer than usual but with marked differences between the west and the east of the country.

Warm nights

Along with the warm days, Sydney had an unusually mild run of overnight temperatures. During summer, 58 nights had minimums of at least 20 degrees, almost triple the average of 22 such nights, and second only to the summer of 1990-91's tally of 64.

The mercury also stayed above 24 degrees on 10 nights, beating the previous record of six such nights, the bureau said.

With nights so mild, days often got off to a warm start. The 36.5 degree reading set at 9am on January 18 was the warmest for that time of day at Observatory Hill since such records began in 1955, the bureau said.

(Australia's roughly 1-degree warming over the past century is in line with global changes, with temperature increases particularly notable in the last four decades, as shown below.)

Rain check

Sydney had a relatively wet end to summer, with the damp conditions set to extend well into the first week of autumn.

Rainfall in February was 205 millimetres, or not far shy of double the average for the month, helping to make up below-average rain in December and January, the bureau said.

Sydney is likely to collect at least 100 millimetres of rain in the current wet spell, with about 10-20 mm likely each day until early next week, Jacob Cronje, a senior meteorologist with Weatherzone, said.

"We are dealing with a very unstable trough," he said, adding that Saturday looks likely to be the wettest of the days on current model predictions.

For the state, summer rainfall was about one-third below average, setting up NSW for an extended fire season.

Nationally, it was the fourth-wettest summer on record, with rainfall 49 per cent above average. (See bureau chart below showing how unusually wet central regions were compared with the east.)

Autumn and El Nino

While parts of eastern Australia are looking at a few days of persistent rain, the Bureau of Meteorology says odds favour a drier and warmer than average autumn for most of the country.

Those conditions may extend further into the year if the bureau's latest updated information on a key driver of Australia's weather is any guide.

Earlier this week, the bureau said the chances of an El Nino event returning later this year have increased, with the probability of one placed at about 50 per cent.

While the autumn is typically a "predictability barrier", six of the eight global models used by the bureau now suggest an El Nino will form by July this year.  (See bureau chart below.)

El Ninos see the typically westward-blowing equatorial Pacific winds stall or reverse. One effect is that rainfall patterns tend to shift eastwards away from Australia and south-east Asia. 

During such events, the Pacific Ocean also absorbs less heat, helping to add an extra boost to the background warming temperatures from climate change.

The 2015-16 El Nino, one of the three biggest on record, added 0.05 degrees and 0.12 degrees to global temperatures in 2015 and 2016, respectively, helping to propel both years to record annual highs, NASA said in January.