The Bureau of Meteorology is forecasting extreme heatwave conditions for much of NSW this weekend, extending into Monday, with temperatures at times exceeding 40 degrees.

"This will increase the fire risk across much of state with Very High and Severe fire danger likely on the weekend. Plan and prepare now," the NSW Rural Fire Service tweeted on Thursday.

"Hot weather is coming. With increased growth, there's an increased risk of grass fires this season. Know what to do to get ready," the RFS said.

Last summer's bush fires destroyed 2476 homes and claimed 26 lives and the RFS said it was important that families take time to prepare in case of a fire crisis.

If in a fire zone households must decide whether to leave early (your safest choice) or prepare to stay.

"Do not wait for an official warning to decide when it is time to leave," the RFS site says.

The BOM Head of Operational Climate Services Dr Andrew Watkins said "even short periods of hot and dry weather increase the risk of fire in summer'.

"This summer heatwaves may not reach the extreme temperatures of recent years, but may be of longer duration and more humid, which can still have a significant impact on human health," he said on Thursday.

The BOM noted the weekend would bring temperatures up to 10 to 16 degrees above the average across the state for this time of the year.

"NSW will not be spared with temperatures climbing easily into the mid 40s for vast swathes of the western part of the state and even into the low 40s for some coastal areas," meteorologist Helen Reid said.

"It will also be quite windy so we are keeping a close eye on the fire danger which is associated with this warm, heatwave situation," Ms Reid said.

"It is going to be quite a volatile situation."

Australian Associated Press