Sydney has just broken a 121-year-old heat record, and it's only day one of the heatwave

Sydney has just broken a long standing summer record for the number of days above 35 degrees in the season.

Temperature readings from the city's main weather station at Observatory Hill are usually cooled by a regular sea breeze in summer, even while western parts of the city bake in 40 degree heat. For this reason, days above 35 degrees are rare between December and February.

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In fact, Observatory Hill averages only three days per year above the 35 degree mark.

The previous record of nine summer days above 35 degrees was set way back in 1895/96 at a site where records extend back to 1858.

As of Friday afternoon, Observatory Hill had exceeded 35 degrees for the tenth time this summer (up to 37.2 degrees as of 2:49pm).

It is highly likely that this record will be extended to 11 days on Saturday with temperatures nearing 40 in the city.

Many other parts of NSW will bake in mid 40's heat over the weekend, before a significant cool change spreads across the state from the south during Sunday and Monday.