Sydney recorded a record high of 45.8 degrees Celsius (114.44 degrees Fahrenheit) in the city, at 2:55pm, beating the previous high of 45.3 degrees set on 14 January 1939.
In the evening temperatures cooled with the arrival of strong winds and rain.
The weather forecast had predicted temperatures of only about 38 or 39 degrees.
Temperatures were even higher at Sydney airport in Mascot where they reached 46.4 degrees at 2.32pm.
The highest temperature recorded in the Greater Sydney area was 46.5° at Penrith.
In western Sydney, the Big Day Out Music Festival was taking place at Olympic Park, where the Red Hot Chilli Peppers were amongst the line up.
NSW Police had four key messages for the public:
• Drink plenty of water (carry some with you);
• Keep cool;
• Take care of others; and,
• Have a plan (know who to contact)
They also advised people not to leave babies, children, the elderly and pets alone in cars.