NSW is set to swelter through its hottest weather in more than a month, with a severe four-day heatwave forecast for the Australia Day long weekend.

At the peak of the heatwave, the mercury is tipped to rise to 42 degrees in Sydney's west on Sunday as a trough from inland Australia brings bone-dry heat until at least Monday.

The four-day heatwave will start on Friday and see temperatures soar in south western Sydney and the state's south, with an extreme heatwave forecast in these parts until Monday.

A severe heatwave will also deliver intense heat to the state's southern half from Broken Hill down to Albury and all the way through Dubbo and Sydney from Friday.

Bureau of Meteorology (BOM) duty forecaster Gabrielle Woodhouse said the heatwave will bring some of the hottest conditions we have seen since late 2020.

"We'll see the heat starting to build across NSW over the course of the weekend... The last time we saw this heat was in early December," she said.

The summer swelter is likely to continue on Australia Day, according to Ms Woodhouse, with temperatures staying in the high 30s until a cool change moves through.

"As we get to Australia Day, we might have a cool trough nearby so either it will stay warm or if the trough comes through it will be cooler," she said.

People are being reminded to stay safe over the weekend due to the risk of heat stroke, particularly among the elderly.

"With the heatwave, it's best for people to check on their friends and family as well as the elderly and vulnerable because the heat takes its toll," Ms Woodhouse said.

Firefighters are also on high alert, with the Rural Fire Service warning the severe weather conditions are "likely to increase fire dangers especially in western parts of the state."

Residents in high-risk areas are being urged to plan and prepare for a bushfire so they know what to do if they are threatened by fire.

Originally published as Australia Day weekend heatwave forecast in NSW