Firefighters are on high alert as the state braces for a scorcher, with sweltering heat and strong winds sparking very high fire danger and igniting grass fires across NSW.

The mercury is set to rise into the low 40s in the Riverina, with powerful gusts reaching 35km/h in the late morning bringing elevated fire danger.

The tinderbox conditions have put the Rural Fire Service on alert, prompting a Very High fire danger in the Southern Slopes, Eastern, Southern and Northern Riverina. A total fire ban is also in place in these parts.

Greg Allan, spokesman at the RFS, said crews are preparing for a high-risk fire day, with large airtankers positioned on the ground in Albury and Richmond if waterbombing is needed.

"The fire risk is higher than we have seen in the last couple of days and weeks. Grass growth has been quite high so the fuel load is high even though crops have been harvested," he said.

"If a grass fire does start they do move incredible fast."

Currently, there are six bush or grass fires burning in the state's south, all of which are under control.

Fire crews are urging residents in high-risk areas to be vigilant, warning fire risk will increase throughout the day as temperatures climb.

"Our main ask of residents across the state is to have a discussion about if a fire does start what they will do," he said.

A southerly buster which is forecast to sweep through the south in the evening could spread flames, he said.

"The humidity and temperatures are of concern, as are the winds. If we do see fires take hold, once we have a southerly, doe shift fire and can cause concern.

Anyone who sees a fire is urged to contact triple-0 and report it immediately.

Originally published as Return of fire danger across NSW as mercury climbs to 40C