The first large heatwave of summer has prompted a fire weather warning and a public health alert with only modest relief forecast for the next few days before the heat starts to build again.

Sydney's expected top temperature was trimmed by the Bureau of Meteorology to 35 degrees for the central business district - reversing an overnight revision higher.

Many western suburbs had already reported temperatures above 40 degrees by early afternoon, with Penrith setting its highest December mark with the mercury topping 43 degrees.

The bureau earlier issued a fire weather warning for the Sydney metro and Illawarra/Shoalhaven fire areas. Both districts have a total fire ban in place for the day.