SYDNEY sweltered through its hottest December night on record as the mercury refused to budge to acceptable levels.

Fans and airconditioners copped a workout and hundreds hit the beach last night as people desperately tried to keep cool.

The Bureau of Meteorology confirmed the overnight temperature dipped to 27.1C.

It was the highest minimum on record. The previous record was set on Christmas Day 1858, when it dropped to 26.3C.

The temperature also smashed all but one previous record, making it the second-warmest Sydney evening in recorded history.

BoM forecaster Jordan Notara told AAP they were expecting last night to break the record of 26.3C..

At 6am this morning the temperature at Sydney Harbour was already 29C and at midnight the temperature was still a warm 27C at Observatory Hill.

The mercury hit 37C yesterday with an expected high of 38C today.

The hot night prompted plenty of reaction on social media, with some remarking it is standard Sydney for this time of year.

#sydney whiners, you get one or two days of 35 plus and you cry. Cut it out sincerely yours Darwin NT #AllYearRound #heatwave — Designs by Legicy (@LegicyDesigns) December 13, 2016

Others tried to see the lighter side of things.

Unless you've got some effing chilli crab for me, stop trying to be like Singapore tonight please, Sydney. #sydneyheatwave — WhamBamThankYouDan (@whambamtydan) December 13, 2016

Bloody hell. Just stepped off train in Sydney's west and into a furnace 🔥ands it's 7pm! #heatwave — Antfarmer (@antfarmer) December 13, 2016

When Sydney folk complain about the heat and you're from Darwin pic.twitter.com/FjLQIQRGGV — Dave Krantz (@weskrantz) December 2, 2016

Can't sleep due to the heat? No surprise. Many areas having hottest night in decades. Stay cool and visit https://t.co/U2KOOC2WF8 #NSWRFS pic.twitter.com/c2j5WGNgEG — NSW RFS (@NSWRFS) December 13, 2016

For those unable to cope with the heat, the good news is a cool change should see the mercury dip to a much more comfortable 22C tomorrow.

“There will be a 5C drop in temperatures within the first few hours, so by early evening people will definitely be feeling the difference,” Mr Notara said.

But be careful for what you wish for though as showers are on the cards for the next few days.

Temperatures will hit 25C on Friday and 31C on Saturday and 22C on Sunday with showers expected across the weekend.

#NSWWeather Hot conditions being replaced by more unsettled weather spreading from southwest, Wed to Sat. https://t.co/nNfJLBJ0Kf pic.twitter.com/AddTkZW7ex — BOM New South Wales (@BOM_NSW) December 13, 2016

Heatwave conditions in parts of #NSWWeather for Tuesday and Wednesday. More info on Heatwaves and Safety Advice at https://t.co/XdmufeEMyr pic.twitter.com/asErO9zCsr — BOM New South Wales (@BOM_NSW) December 12, 2016

But it wasn’t just Sydney which felt the heat yesterday.

A blast of hot weather across Australia’s southeast also left Canberra, Adelaide and Melbourne all sweltering together for the first time in more than half a century.

Temperatures peaked above 33C in all four capital cities yesterday — the first time in 51 years, according to the Bureau of Meteorology.

Hot, dry winds drawn down from Northern Australia drove the mercury higher, sparking fire bans in NSW, South Australia and Victoria.

Sydney’s CBD sizzled at a high of 37.8C — nearly 13 degrees above the city’s long-term average for December — while at Penrith, in the city’s west, the mercury surged to 39.4C.

About 800 people had flocked to the Aquatic Centre at Sydney Olympic Park in the city’s west before lunchtime to seek relief from the city’s hottest December day since 2005.

In Melbourne, temperatures leapt above the forecast maximum of 34C to reach 36.1C shortly after 4pm.

Adelaide was a touch hotter at 35.6C, while Canberra hit 33.9C.

Cool winds brought relief for Melbourne with temperatures hovering between 20C and 23C for the next few days.

Adelaide is also tipped for a cooler day today with a high of 23C expected today and 25C tomorrow.

Hot conditions in SE states. Cloud band stretching from WA, across SA, Vic and Tas ahead of "cooler" wind change!https://t.co/DO9KX5WTyA pic.twitter.com/cGuhg1Nyz5 — BOM Australia (@BOM_au) December 13, 2016

FIRE DANGER

Meanwhile, around 40 fires were still burning across NSW, including two large blazes which threatened dozens of homes near the towns of Abermain and neighbouring Neath, near Cessnock in the Hunter Region.

At least 70 firefighters were on the ground while water bombing helicopters helped from above.

With hot and windy conditions forecast to continue, total fire bans have been declared for the greater Sydney region, Illawarra/Shoalhaven, Upper Central West Plains and Lower Central West Plains today.

However, fire bans that were in place across northern Victoria and most of South Australia are expected to be lifted today.

The SA Country Fire Service said that the threat posed by a fire on 3300 hectares of land near the Nullarbor Roadhouse on the Eyre Highway had reduced.