Sweltering day leaves crews with 70 blazes

Fire crews are dealing with almost 70 fires around NSW after a day of sweltering temperatures and lightning strikes.

Richmond northwest of Sydney was among the hottest places to be on Sunday when it hit 45.3C.

Penrith, west of the city, reached 44.1C as a hot air mass moved across the state.

Meanwhile further east a blanket of sea mist covered Sydney's coast until early afternoon.

In a fairly rare event, cool humid air formed a thick sea mist from Botany Bay to the northern beaches.

A band of lightning across the New England and central regions sparked 20 new fires on Sunday afternoon.

"We have around 70 fires burning across the state, including 37 uncontained," Rural Fire Service (RFS) spokesman Matt Sun said.

One fire at Peak Hill in the central west was at a watch and act alert level on Sunday night and had burned 2500 hectares.

A total fire ban will be lifted for most parts of the state on Monday, except the lower central west plains.

The heat also played havoc with power supplies, prompting a blackout across 1000 homes in Castle Hill and Cherrybrook on Sunday.

Showers and a possible thunderstorm are forecast for Sydney on Monday with a maximum temperature of 30C.