Sydney is predicted to endure more unseasonably hot weather after sweltering through its hottest April day on record.

Monday reached a top of 35.4 degrees at the Observatory Hill city weather station, eclipsing the previous record of 34.2 set just two years ago.

The Bureau of Meteorology (BoM) said temperatures at the airport hit 36.8, making it the hottest April day since records began at the station 88 years ago.

A weak cool change is expected to bring a brief reprieve for Tuesday, bringing the maximum temperature down to 24 degrees.

“But that doesn’t last very long,” Weatherzone forecaster Kim Westcott told The New Daily.

“The cool change isn’t really that strong.”

The BoM is forecasting Wednesday to reach 28 degrees, before tops of 31 on both Thursday and Friday.

“It’s a significant departure from average temperatures that we would expect during April, and it has been since the start of April quite warm,” Ms Westcott said.

The long-term April average is just 22.5 degrees, making Monday about 12 degrees above average.

Very hire fire danger today for Central Ranges, Southern Ranges, North Western and Greater Hunter. Postpone any burning activity. pic.twitter.com/4QVF0ccZ0Q — NSW Incident Alerts (@nswincidents) April 9, 2018

More relief is expected towards the end of the weekend or early next week with a “nice big cold front”. Temperatures will likely remain warm, but will be a break from the high-20s and 30s.

The heatwave is impacting much of New South Wales, as well as Victoria and South Australia.

“Sydney is not alone in this very un-April-like weather,” Ms Westcott said.

Adelaide hit 34 degrees on Sunday, 35 on Monday and is forecast to reach 34 degrees on Tuesday. If that’s achieved, it will be the first time Adelaide has ever had three consecutive days over 33 degrees in April.

Southern parts of the state were also tipped to reach 40 degrees.

Ms Westcott said the above-average temperatures were the result of “lingering” heat that had built up over Western Australia.

NSW Incident Alerts warned of a very high fire danger on Monday for much of the state.

In its annual climate statement released in January, the BoM said climate change had driven the Australian annual mean temperature up by 1.1 degrees.

Australia experienced the third-warmest year on record in 2017.