Sydney has sweltered through the hottest December night in 159 years, breaking weather bureau records, after temperatures stayed above 27C overnight.

The previous highest minimum December overnight temperature of 26.3C was recorded almost 150 years ago, in 1868.

Weatherzone, which calculates records based on temperature readings taken up until 9am, told 9news.com.au it was also the second hottest night on record in 159 years of records.

The only hotter night was in February 2011, when the minimum overnight temperature was 27.6C.

Just after 6am the CBD was sitting just above 27C and temperatures ranged from around 23C to 28C across the greater Sydney area.

Crowds cool off at Bondi Beach today. (AAP)

BoM's heatwave map for December 12 to December 14. (BoM)

The city is expected to swelter today, with a scorching high of 38C forecast after yesterday’s peak of 37.8C.

Meanwhile, the NSW Rural Fire Service has placed total fire bans on the Greater Sydney, Illawarra/Shoalhaven, lower and upper Central West regions.

Severe fire danger warnings are also in place for Sydney and the Illawarra.

Yesterday, the hot weather left Sydney, Canberra, Adelaide and Melbourne residents all sweltering in temperatures above 33C for the first time in more than 50 years.

Sydney is facing its hottest back-to-back days in 10 years. (AAP)

The last time temperatures peaked above 33C in all four capital cities was 1965, the BoM said.

The hot weather is being driven by warm, dry winds drawn down from northern Australia.

But a cool change is on the way for Sydney later today, while Melbourne, Canberra and Adelaide can already expect some relief.

Read more: Bushfire threatens homes in NSW Hunter Valley

Yesterday, Sydney sweltered though its hottest December day in 11 years.

Temperatures peaked at 37.8C in the CBD about 1pm and 39.9C in Penrith at 1.30pm, before an afternoon north-easterly sea breeze cooled the coast down.

Today, a record for Sydney’s hottest back-to-back days in a decade could be set, with the mercury forecast to again climb to 39C before the forecast cool change.

The Red Cross has urged people to keep an eye on their neighbours.

"Older people, pregnant women, children, those with a disability and people taking medications are among those who are more at risk," a spokeswoman said.