High temperatures, combined with high humidity, made it an unpleasant sleep for some in Sydney overnight.

The Bureau of Meteorology confirmed on Monday that Penrith had reached the highest temperature on earth in the past 24 hours when it reached 47.3 degrees on Sunday afternoon.

"It looks like it's the highest temperature recorded in the Sydney area in 80 years," Jacob Cronje, a senior meteorologist with Weatherzone, said.

"It was certainly the hottest place in Australia," he said.