Temperatures will top 40 degrees in many parts of NSW this week.
Towns beyond the Great Dividing Range are likely to experience extreme heat as early as Monday.
Residents in Sydney's western suburbs might be tempted to head to the beach by mid-week, with the mercury in Penrith expected to hit the high 30s from Wednesday.
Respite from the heat won't be felt until Saturday, when there will be a cooler southerly change.
Weatherzone meteorologist Sam Brown said those on the coast will be spared record temperature highs, with sea breezes keeping temperatures down.
Temperatures for the Australian Open in Melbourne will be about 40 from Tuesday to Friday.
Sydney's beaches were packed on Sunday. Several thousand members of Sydney's Greek community turned out for the Greek Orthodox Church's celebration of Epiphany at La Perouse.
The event is capped by the holy cross event, this year taken out by Theodore Pasialis, who was the first of 150-odd young men to retrieve a cross thrown into the water.