Sydney just reached 40 degrees for the first time in two years and the heat will last well into the night.

A mass of hot air being driven over NSW ahead of a cold front is causing a sweltering day across the Sydney Basin on Thursday.

The temperature in the city had already risen to 30 degrees by 8:30am and reached 40 degrees shortly before 1pm. This was the city's first day over 40 degrees since January 2018.

Image: Observed temperatures in the Sydney Basin at 1:10pm AEDT on Thursday.

Unlike many of Sydney's hottest summer days, the highest temperatures in the early afternoon were being registered close to the coast in the absence of a sea breeze. Sydney Airport reached 42 degrees just before 2pm, one degree warmer than Penrith at the same time.

Temperatures will peak across the Sydney Basin on Thursday afternoon, although it's going to stay hot well into the night.

Computer models suggest that mercury could still be up around 30 degrees at midnight, before a southerly change arrives in the early hours of Friday morning.