"It's a great day for shorts," Brett Dutschke, a senior meteorologist at Weatherzone, said.

The balmy day follows an unusually mild night, with the mercury dipping to just 17.2 degrees at 5am, more than 9 degrees above the long-run average minimums in July for the city.

David Martin, a senior climatologist at the bureau, said that Sydney has only posted two consecutive July days of more than 24 degrees in 1975, 2014 and 2016.

Averaging the two days, this week's warmth was just 0.1 degree behind the 1975 record.

A typical July maximum is 16.4 degrees, while the average December top in the city is 25.2, according to bureau data.

A steady build-up of heat over north-western and inland Australia in the past week, combined with a weakening of the frontal activity across southern Australia, is a factor in the abnormal heat across south-eastern Australia. Powerful north-westerly winds have pushed that heat over the ranges into Sydney, Mr Dutschke said.

Sydney's 25.2 degrees on Thursday was the hottest for this early in the warming season.

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Early-season high temperature records were also exceeded at Cobar, Broken Hill Airport, Griffith Airport, Wagga Wagga and Camden Airport, Mr Martin said. Early-season records were also exceeded at sites in Victoria and South Australia.

However, the unusual warmth won't last far into Friday afternoon, with winds expected to start to shift around towards the south-west by about mid-afternoon.

"It will cool down fairly quickly," Mr Dutschke said, adding that sub-20 degree conditions should be widespread by about 4pm for much of the city.

The weekend should see normality of sorts return, with dry south-westerly winds "on the chilly side", he said.

On current forecasts, Saturday and Sunday's tops of 17-18 degrees will be the most likely range for much of the coming week, with little or no rain.

The snowfields will fare better, receiving about 30 centimetres of fresh snow on the main resorts, helping to make up for the recent melt, and then some, Mr Dutschke said.

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