With temperatures predicted to reach 41 degrees, some stores in the Highlands have sold out of fans.

Big W has officially sold out, while Bing Lee in Bowral is trying to source more stock but has already sold out a few times this summer.

Harvey Norman at Moss Vale has also sold out several times over the summer but currently has limited stock of pedestal and desk fans.

With a couple of cool and rainy days early in the week, the mercury will rise from Thursday with a top of 31 degrees.

Tuesday was the coolest day this week with a top of just 19.3 degrees.

The temperature will continue to increase over the weekend with Friday and Saturday the hottest days of the week with a top of 39 and 41 degrees respectively.

Meteorologist Ashleigh Lange from the Bureau of Meteorology said the extreme heat on Friday and Saturday would be caused by a warm air mass over the whole of NSW.

She said a cool change should come through on Sunday afternoon and provide Highlands residents with some relief after a warm night on Saturday.

The mercury on Monday will peak at 21 degrees.

So far in February, the average temperature has been 28.4 degrees compared to 25.7 degrees in 2016.

For January the average temperature was 28.8 degrees.

In 2016, this average was 24.2 degrees.

The NSW Department of Health has released some information to help people stay safe in warm weather,

The department advises people to drink plenty of water, keep cool and look after others.

For people who will be outside over the weekend, as well as drinking plenty of water, Cancer Council has plenty of sun smart tips: