Sydney is about to experience two unseasonably warm days, possibly climbing within two degrees of the city's July maximum temperature record.

A stream of northwesterly winds ahead of an approaching front will carry warm air from central Australia across NSW during the next 48 hours.

The inland air mass will cause daytime temperatures to climb into the low to mid twenties across the Sydney Basin on Thursday and Friday. While this isn't hot for Sydney by summer standards, its unusual for the middle of winter.

The highest July temperature recorded at Sydney's Observatory Hill during the last 159 years was 26.5 degrees in 2017, while the average maximum for this time of year is just 16 degrees.

Cooler air will arrive in Sydney with a southwesterly wind change late on Friday, pegging maximum temperatures back into the teens for the weekend.