Paramedics have warned heatstroke "can be a killer" after a man collapsed and died on a golf course during sweltering temperatures in Sydney this afternoon.

The man was at Woolooware Golf Club at about 12.48pm on Wednesday when he collapsed in the heat and went into cardiac arrest.

At the time, a nearby weather station recorded the temperature to be 38 degrees.

Golf club staff and paramedics were unable to revive the man, aged in his 60s, who was one of dozens of people needing an ambulance after they were overcome by heat.