Meteorologists predicted the strong cool change would hit the city from the south at about 5am on Wednesday, and that's precisely when it swept over Sydney Airport, Anthony Duke, a meteorologist with Weatherzone, said.

Minutes later, as shown in the Doppler radar image in the video above, the strong winds had reached Sydney Harbour, buffeting any boaties mad enough to out for an early sail.

The Bureau of Meteorology has since cancelled its severe weather warning for damaging winds that it had issued late on Tuesday for a region stretching from the south coast up to the Hunter district

Wattamolla, on the cliffs of the Royal National Park to Sydney's south, recorded the day's strongest winds of 95 km/h, also just before 5am.

Around Sydney, the harbour recorded 76 km/h and Norah Head up the coast collected 78 km/h gusts, the bureau said.

"The winds will gradually ease through the day," Mr Duke said, adding that the haze, dust and pollen kicked up by the change will start to settle.

Unlike in Melbourne, where the cool change sent temperatures tumbling about 13 degrees in just eight minutes, Sydney's buster arrived around sunrise, leaving the mercury hovering just below the 20-degree mark.

The bureau defines classic southerly busters as bringing wind gusts of at least 54 km/h and a temperature drop of at least five degrees over three hours.

Instead, maximum temperatures for the city may remain close to the 21.5 degree peak reached just after 9am for the most of the day, with a similar top forecast for Thursday.

Sydney typically receives about five such abrupt changes, also known as southerly busters, each year, the bureau said.

Tasmania, New Zealand and even Argentina lay claim to similar events.

In Sydney's case, cool air gets trapped against the Great Dividing Range, intensifying the severity of the winds. A "roll cloud" often accompanying the change as it moves north.

Wednesday's return to more typical October daytime temperatures follows Sydney's hottest trio of days this early in the warming season on record, according to Weatherzone.

For any time of the month, the three days were hottest in October since 2004, Mr Duke said.

NSW fire authorities are not expecting the gusty winds to cause major flare-ups among the 15 fires still burning uncontained in the state as of Wednesday morning, a NSW Rural Fire Service spokesman said.

The RFS dispatched two of its main firefighting aircraft from Richmond air base to Victoria for a single drop of fire-retardants on Tuesday, the spokesman said, adding the planes remain ready to fly south again if needed by Victorian counterparts.

The string of record-breaking heat across south-eastern may prompt many in NSW to consider their fire plans.

"This may be the trigger they needed to think about the fire season," the spokesman said. "We're in the fire season and we need to be prepared."

The powerful El Nino developing the Pacific - already one of the most intense on record - is combining with Indian Ocean conditions that have become less conducive for rainfall over the Australian continent.

Last month was the country's third-driest September on record, with only about one-third of the typical rain falling across the nation.

Weatherzone is owned by Fairfax Media, publisher of this website