Hottest April day on record hits Sydney

Sydney has broken the record for April heat as the mercury topped 34.2C in the city.

The hottest April day on record beat the previous high of 33.9C set in 1986, while temperatures in Penrith and Richmond reached 36.6C on Wednesday, according to the Bureau of Meteorology (BOM).

"Record-breaking temperatures have come to a few parts of the state, including Sydney and the south coast where the mercury reached 35.4C," BOM forecaster Sarah Chadwick told AAP on Wednesday.

The heat is coming from the outback as a high pressure system on the Tasman Sea directs northerly air down, she said.

A cool change is expected to sweep through the city from 11pm and temperatures will drop through to the low 20s in coming days.