According to the Bureau of Meteorology (BOM), the temperature reached 20.8 degrees, the hottest temperature at that time and day in two years.

Senior climatologist Blair Trewin said the temperature was 4.2 degrees higher than the next highest temperature, which was reached on August 24, 2009. “The peak of 21.0 reached in the early hours of this morning was in fact only 1.5 short of the August record maximum for Moss Vale and makes it the warmest August "day" for two years,” Mr Trewin said.

Mr Trewin said the temperature was consistent with the overall warming trend in Australia. “As an individual event this is clearly an exceptional event,” he said.

“However, the occurrence of extreme high temperatures both day and night has an increasing long-term trend through most of Australia.”

Mr Trewin said the general warmth was caused by west/northwest winds from central and northern Australia.

“This is amplified because it has been an exceptionally warm winter in those regions (July was the warmest on record for most of northern Australia), so the pool of warm air that northwesterly winds can draw on is hotter than in a normal year,” he said.

“The very warm overnight conditions were also the result of the strong winds.

“On calmer nights, heat radiates away from the ground and the layer of air closest to the ground is cooler than the air a few hundred metres aloft, but on windy nights that doesn't happen and the overnight temperature stays close to where it was during the day.”

In Moss Vale, the temperature dropped to 11.9 degrees at midnight when winds were light, but jumped to nearly 20 degrees by 1am as the wind strengthened.

“The warmth in Moss Vale looks to have been abnormal even compared with other sites in the region, so there may well have been some interesting local effects going on, but that will require a lot more investigation,” Mr Trewin said.

The temperature at High Range was about a degree cooler than Moss Vale, but behaved similarly, with temperatures in the 19s from 12.30am to 3.30am. High Range was windier than Moss Vale, which accounted for a temperature dip around midnight.

There was also a sever weather warning for vigorous winds issued for the Southern Highlands.

Damaging winds averaging 60 to 70 km/h, with gusts in excess of 90 km/h, were predicted from late Wednesday morning until the evening. SES Wingecarribee Unit media liaison Carly Miller said the unit had six referrals, all of which were passed on to council and a tree arborist.