Nine people were admitted hospital on Saturday with heat stress, stroke and exhaustion, as parts of New South Wales experienced temperatures of almost 43 degrees.
AAP reports another three were treated by ambulance staff.
Friday night was the hottest start to summer in Sydney since records started in 1859.
The Bureau of Meteorology said the city experienced a minimum of 23 degrees overnight.
The hottest place in the state on Saturday was Coonamble in NSW's central west, which reached 42.8C.
Despite the drop in temperatures, meteorologist Julie Evans said humidity is expected to linger across Sydney until mid-week.
"On Wednesday, a westerly change is coming through and taking a lot of that humidity away," she told AAP.
However, early forecasts show there could be high temperatures back across the state next weekend. A top of 31 degrees is forecast for Sydney next Saturday and a maximum of 37 degrees in the western suburbs.
Ms Evans said it was shaping up to be a warmer summer than last year.
"Last summer, only two days reached 30C at Observatory Hill (in Sydney)."