Sydney is heading for its hottest ever January for overnight temperatures with little sign the unusual warmth across eastern Australia is about to lose its grip.

Temperatures are forecast to nudge or exceed 40 degrees on Tuesday, with Richmond tipped to reach 41 degrees. The town on Sydney's north-west has already had a record-equalling five days above 40 degrees this month.

By just before 10.30am, the city had already reached 36.9 degrees, while Sydney Airport was the hottest site in NSW with 38 degrees.

Sydney has now reached or topped 35 degrees eight times this summer, the equal second-most on record, and matching the 1979-80 season. With more than a month of summer to go, the record of nine such days - set back in 1895-96 - is in range.