Sydney sweltered through its hottest early September day in 159 years on Wednesday as the temperature reached 33.8 degrees in the city at 2.30pm.

It was also the fourth-warmest reading for any September day.

A hot and windy day saw fire risks lifted to extreme in parts of Sydney and elsewhere in the state. The NSW Rural Fire Service said about 1000 firefighters were dispatched to fight 94 bush and grass fires that were burning around NSW. Thirty eight remain uncontained.

A bushfire emergency warning issued for Richmond Vale near Cessnock in the Hunter where a number of properties are under threat.