Sydneysiders can soon breathe a sigh of relief as the weekend's heatwave will give way to milder spring days early in the next week.

After sweltering through record-breaking temperatures on Saturday night, Sydneysiders woke to another sizzler on Sunday - with the mercury heating up to 28 degrees by 6am.

Despite high cloud cover, the temperature has remained high throughout Sunday, reaching a maximum of 29.2 degrees in the CBD on Sunday afternoon - 9 degrees above September averages.

Meanwhile, regional areas of NSW faced a third consecutive day of sweltering temperatures.