Overnight conditions were actually cooler in the city's west, where Penrith dropped to a low of 25.3 degrees and Badgerys Creek to 23 degrees in the early hours of Wednesday morning.

The overnight temperature observations for the city made for some uncomfortable reading, and explain why many Sydneysiders would have tossed and turned into the early hours.

At 10pm on Tuesday, the temperature at Sydney's Observatory Hill was still sitting at 32.3 degrees.

An hour later, the city hit its minimum overnight temperature of 28.1 degrees, before it began climbing again, cracking 30 degrees at midnight.

Those waking up in the city at 6am were confronted with 31.2-degree conditions.

"We have a very warm air mass over us at the moment and some light to moderate winds that helped to keep that warm temperature at the surface," said Miranda Langton, a duty forecaster with the Bureau of Meteorology.

"It's been hot across the whole of NSW. Cobar is still sitting on 31 degrees [just before 6am on Wednesday]. Even somewhere like Thredbo is about 16 degrees." The average minimum temperature for Thredbo's top station for January is 6.7 degrees.

Ms Langton said that, despite the stifling night-time conditions, Sydney technically did not break the record for its hottest ever January night. That was recorded in 1994, when the temperature reached a minimum of 26.2 degrees.

Daily temperature observations are recorded over a 24-hour period to 9am and, at 9am on Tuesday in the city, it was 25 degrees.

"A similar thing happened [on Friday] night. We thought we'd broken the record but the 9am temperature was lower," Ms Langton said.

Sydney on Tuesday climbed over 35 degrees for the sixth day this summer, the first time that has happened since the summer of 1990-1. The record of the most summer days over 35 degrees was set in 1895-6, when nine days were recorded over that temperature.

That record could tumble this year. On Wednesday, the city is forecast to reach a maximum of 37 degrees, while Tuesday next week is forecast to reach 38 degrees, and the city is only half way through summer.

The long-term average of Sydney summer days over 35 degrees is just two per season.

Sydney's west is expected to be even hotter on Wednesday, with 42 degrees forecast in Penrith and Richmond.

Bleary-eyed residents can take solace in the fact that a cool change is slowly moving its way up the NSW coast.

Ms Langton said that cool change should move through Sydney on Wednesday morning, with a medium chance of showers in the early afternoon and late evening, and the chance of a thunderstorm in the early afternoon.

Conditions are expected to be much more comfortable for sleeping on Wednesday, with a low of 21 degrees forecast in the city, rising to a maximum of 25 degrees on Thursday.

The Rural Fire Service has declared a total fire ban on Wednesday for the Greater Hunter, Greater Sydney, Illawarra/Shoalhaven, Southern Ranges, Central Ranges and Southern Slopes regions of NSW.

On Tuesday, one home was destroyed by a fast-moving and out-of-control bushfire near Canberra, on a day when temperatures soared across the state.

The blaze, referred to as the Currandooley fire, also threatened homes at Boro and Mount Fairy, near Tarago.