The record for Australia's all-time highest overnight temperature was smashed last night, just one of the marks to fall in what is becoming a heatwave for the history books.

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Dr Blair Trewin, senior climatologist at the Bureau of Meteorology, said a minimum of 35.9 degrees Celsius was recorded at Noona, between Wilcannia and Cobar in western New South Wales, while 35.6C was measured at Borrona Downs.

"Both of those broke the previous record which was 35.5C set in 1982 in Arkaroola, South Australia, and was equalled in 2003 at Wittenoom in WA."

He said the current heatwave was shaping up as one of the most significant on record in inland New South Wales.

"We have now seen five days in a row with temperatures widely well into the 40s, and probably the only event that's comparable on a statewide level was the January 1939 heatwave.

"While the 1939 heatwave was a bit longer in New South Wales, this one has been much more extensive nationally so it's certainly one of the most significant heatwaves we have seen."

Highest minimum temperatures (before last night) Rank °C Date

Station name State 1 35.5 January 21, 2003 Wittenoom WA 35.5 January, 24 1982 Arkaroola SA 3 35.4 January, 24 1982 Andamooka SA 35.4 January 24, 1982 Port Augusta Power Station SA 5 35.2 January 24, 1982 Adelaide (Waite Institute) SA 35.2 January 24, 1982 Adelaide (Parafield Plant Introduction Centre) SA 7 35.1 February 21, 2015 Wittenoom WA 35.1 January 24, 1916 Marble Bar Comparison WA 9 35.0 February 17, 2004 Moomba Airport SA 35.0 January 24, 1982 Adelaide (Black Hill Conservation Park) SA 35.0 January 24, 1982 Belair (Kalyra) SA 35.0 January 30, 1973 Marree Comparison SA 35.0 November 6, 1965 Cunnamulla Post Office QLD 35.0 December 28, 1902 Marble Bar Comparison WA Source: Bureau of Meteorology

How widespread is the heat?

It wasn't just hot in Noona.

"There were widespread high minimum temperatures throughout outback NSW and north-east South Australia," Dr Trewin said.

"There were several other sites that had minimum temperatures over 33C.

"The likes of Cobar airport, Fowler's Gap north of Broken Hill, Tibooburra and White Cliffs were all in the 33s and 34s."

Why so hot last night?

While the heatwave has been going on for days now, Dr Trewin said there were a few reasons why records fell on Thursday night.

"The thing that made last night a little bit different was there was just a little bit of high cloud, which kept some of the heat of the day in," he said.

"Also in the areas that got the highest temperatures, there was a bit of wind as well.

"When there is wind, that stops relatively cool layers of air forming close to the ground, so windy nights tend to be warmer, all other things being equal."

Part of a dangerous trend

Hot nights can be dangerous because the body can't recover from scorching daytime temperatures and it prevents people getting a restful sleep.

Internationally, it has been observed that night-time temperatures are increasing at a faster rate than daytime temperatures.

Minimum temperatures have been increasing in Australia for the past 50 years, and according to the CSIRO the number of warm nights is expected to continue to rise.

Nights where minimums will stay over 25C in 2075-2104 under ACCESS1-0 model at RCP 8.5 (high emissions) scenario, December to February. ( Supplied: CSIRO Thresholds Calculator )

When will it end?

The general long-term heating isn't going to be over any time soon, but this latest burst is expected to ease over the weekend before ramping back up.

"It looks like [Friday] is probably going to be the last day in this phase of near-record high temperatures," Dr Trewin said.

"We are seeing a bit of a change move across south-east Australia at the moment."

He said this change had gone through Melbourne and Adelaide and was expected to go progressively across NSW over the next day or so.

"But it's only a weak change and temperatures in inland NSW particularly will still stay significantly above average.

"For example, north-west NSW will only drop daytime temperatures from the mid to high 40s, where they have been this week, down to the low 40s."

That's not all; it looks like it could heat up again next week.

"There are some indications of another round of extreme heat towards the end of next week," Dr Trewin said.