A golfer has collapsed and died in Sydney as temperatures soared in the city before plummeting by almost 20C as thunderstorms approached.
Parts of Sydney reached 43C on Wednesday before the cool change caused temperatures to drop into the mid-20s and delivered rain, thunder and lightning.
Earlier, paramedics tried but failed to save man in his 60s who collapsed at the Woolooware Golf Club before 1pm.
"It appears the man collapsed in the heat just after lunchtime ... and went into cardiac arrest," Chief Superintendent Alan Morrison said in a statement.
"This is a terrible tragedy."
NSW paramedics were called out to more than 50 heat-affected patients which they said was "a major cause for concern".
Authorities stressed that in extreme heat people should stay inside, drink water and avoid physical activity.
"People need to understand heat stroke can be a killer and not take their health for granted," Supt Morrison said.
Penrith exceeded 40C early in the afternoon but the cool change dropped temperatures by close to 20C in just two hours, Bureau of Meteorology forecaster Gabrielle Woodhouse told AAP.
"What we've had is a cool front that has been moving across the state and reached Sydney early in the afternoon," Ms Woodhouse said on Wednesday.
"With that we saw the temperatures drop pretty quickly."
The NSW Rural Fire Service issued total fire bans for the Greater Sydney Region, the Hunter Region and North Western NSW, which have severe fire danger ratings.
Thursday is expected to be cooler in Sydney, with temperatures in the mid-20s.
