The mercury will peak at 34 degrees in the city and 41 degrees in the west on Sunday and humidity levels will also increase throughout the day, senior meteorologist at Weatherzone, Jacobus Cronje said.

By 1pm, temperatures had reached 32.3 degrees in the city and 39 degrees in Penrith.

However, showers are expected to begin in the early afternoon in south-west and western Sydney, with the possibility of some rain in the city.

"It could be the same scenario as we had the other day with more lightning and thunder in the south-west and the city getting some lightning," Mr Cronje said.

"There will be localised heavy falls and gusty winds ahead of the cool change."

Temperatures will fall significantly between 5pm and 7pm on Sunday to between 22 and 25 degrees, the Bureau of Meteorology said. The drop is expected happen within an hour or so, and will hit coastal areas along the south and east before it does the western suburbs.

The temperature will then drop to about 20 degrees overnight.

"It'll be very windy throughout the evening," Mr Cronje said.

"It will still be quite humid and there will be a lot of cloud cover overnight."

Christmas Day will be much cooler than Sunday, with showers in the morning and the mercury expected to peak at about 23 degrees in the city and 26 in the west.

However, those hoping to have Christmas lunch outside most likely won't have to change their plans, with the rain expected to clear up before midday.

"Showers should be gone by 11am in the city and should clear up earlier than that in the west," Mr Cronje said.

For those out and about on Sunday, heavy traffic is expected around the Sydney Fish Market in Pyrmont throughout the day.

There is also heavy traffic at Sydney's international airport, with people advised to allow plenty of extra travel time.

Weatherzone is owned by Fairfax Media, publisher of this website.