Hot and dry conditions throughout the week will mean temperatures won't dip below 20 degrees, even in the cooler parts of the day, the Bureau of Meteorology forecasts.

"Having temperatures above 21 or so puts it in the hottest 10 per cent of nights for January. It's not totally out of the ordinary but it is unusual," said Bureau of Meteorology forecaster Rose Barr.

The normal average minimum temperature for January is around 19 degrees, but as heatwave conditions worsen later in the week that number will stay as high as 26 degrees.

"From Wednesday through Friday we're going to start seeing intense heatwave conditions in the southern parts of NSW, and that increases into an extreme heatwave at the end of the week in terms of both minimum and maximum temperatures," Ms Barr said.