The delay of an hour or two meant temperatures remained in the 30s as commuters travelled home - barely cooler than the morning's journey to work.

The southerly buster "stalled when it rounded the south coast", Tristan Meyers, a meteorologist with Weatherzone, said.

The winds have shifted to the south-east at Wattamolla in the Royal National Park, reaching 61 km/h at 4.43pm, the Bureau of Meteorology said.

Temperatures in coastal towns dropped by 10 degrees or more in an hour as the gusty change moved through.

By 6pm on Thursday, the temperature at Sydney Airport had dropped to 20.6 degrees.

The southerly change swept through the area bringing with it a 13 degree temperature drop in the space of an hour.

Thursday's top of 37.8 degrees was Sydney's second day in less than a week to break the 100-mark in the old Fahrenheit gauge. Last Friday's 40.9 degrees was the city's second-hottest November on record.

Hot spots on Thursday included 39.2 degrees at Sydney Airport and 41.6 degrees in Willamtown to the north.

The city's top was matched in Penrith, one indication of how uniform the heat was across the Sydney Basin as sea breeze had little influence.

Sydney has had six days so far this month above 30 degrees - or triple the long-run average. The mercury is not expected to break the mark again until Tuesday, the start of December, according to Bureau of Meteorology forecasts.

The rest of the month should see more moderate days, with tops of just 23 predicted Friday and 24 degrees for Saturday.

As with last week's change, little rain is expected before temperatures pick up again. Concern about another day with hot north-westerly winds blowing in from inland regions prompted the Rural Fire Service to declared total fire ban day on Thursday for the Greater Sydney and Hunter as well as the north-western and northern slopes regions.

The RFS was reporting 30 bush and grass fires in the state, sending in nine aircraft and 120 trucks to battle the blazes.

The Bureau of Meteorology said Australia is on course to record its sixth hottest year on record in 2015. Should the last two months of the year matched the warmest November-December period, the ranking will rise a notch to fifth-warmest.

Blair Trewin, the bureau's senior climatologist, the full impact of this year's super El Nino in the Pacific on worldwide temperatures may be yet to come.

"The El Nino effect on global temperatures is typically strongest from about September of the first El Nino year to August of the following year," Dr Trewin said.

"One can think of the first part of 2015's global temperatures as being driven by the 2014 'almost El Nino', with the influence of the 2015 El Nino only just starting to kick in over the last couple of months."

The World Meteorological Organisation on Wednesday declared that 2015 will be the hottest year on record, and 2016 may set the annual bar higher still for a third year running.

Dr Trewin said El Ninos typically add 0.1 to 0.15 degrees to annual worldwide temperatures.

The boost comes on top of the background warming from climate change. Since 2000, records in Australia for daily heat are 12 times more likely to be set than cold day records, the bureau has said.