Residents in Sydney's west are advised to stay indoors throughout sweltering temperatures on Saturday, before late afternoon relief arrives in the form of strong southerly winds.

"At this stage, we're looking at a cool change in the late afternoon as that southerly buster arrives on the coast," said meteorologist Rob Taggart.

"We're talking about a drop of 20 degrees in the space of three hours. By the time we get to the evening, we'll see temperatures in the low 20s."

Tops are expected to hover in the mid-30s with a maximum of 36 near the coastline rather than reaching 40, with sea breezes arriving earlier than previously expected. The forecast for Penrith and Richmond remains at 47 degrees and is expected to peak at 3pm, before rapidly cooling off.