Damaging winds and large hailstones were on Monday forecast for areas stretching from Cooma in the south to Wilcannia in the far west, with the system then pushed northeast by a cool evening southerly.

Greater Sydney could also experience thunderstorms over the course of Monday evening but they would likely subside around 8pm.

"As that change moves through, that's really triggering that thunderstorm activity and also those warm conditions ahead of that change," Bureau of Meteorology NSW manager Agata Imielska said.

NSW's State Emergency Service has recommended those preparing for thunderstorms move cars undercover, secure loose items, stay indoors and keep eight metres away from fallen power lines.

Meanwhile, the state's northern districts are facing an elevated fire threat amid hot and windy Monday afternoon conditions. The Far North Coast and North Western regions are exposed to "severe" fire danger.

A fire in the Glenugie state forest, 18km southwest of Grafton, was on Monday elevated to "watch and act" level after burning out of control.

The Rural Fire Service is sending additional aircraft to fight the blaze.

Monday's cool southerly may also trigger thunderstorms as it passes over the state's northeast, Ms Imielska said. She recommended those in the northwest and northeast keep an eye on fire conditions.

Difficult fire conditions could then return from Thursday to Sunday as a fresh heatwave rolls across the state, particularly western NSW.

Australian Associated Press