It is going to be hot and sticky in Sydney over the weekend, with temperatures set to hit the mid-40s by Sunday.

On Saturday, Richmond and Penrith are forecast to reach 41 and 42 degrees respectively before hitting tops of 44 and 45 degrees on Sunday.

While Saturday is a little milder in the inner city - a top of 31 in the city, and 34 degrees at Mascot - it will be hot right across greater Sydney on Sunday.

Weatherzone meteorologist Graeme Brittain said a high of 43 degrees was forecast for both Liverpool and Blacktown on Sunday, and there was potential for the inner city to also hit 40 degrees.