The mercury in Sydney's CBD may shoot 12 degrees over the historical average for December this weekend while the western suburbs face their hottest day ever in the month.

During the first half of the week, it will be hotter in the central parts of the country.

Temperatures in Sydney could reach 40 degrees on Saturday. Credit:Kate Geraghty

But Sydney will cop the brunt of the heat in the latter half, with the Bureau of Meteorology forecasting a top of 46 degrees in Penrith on Saturday, which would be the highest temperature ever recorded in December.

"If Penrith gets shrouded by bushfire smoke, it may not get that warm - but either way we're pretty confident of getting temperatures well into the 40s," meteorologist Rob Taggart said. He noted that in some parts of western Sydney, measurements only went back 25 to 45 years.