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The oppressive humidity of late has only made temperatures less bearable, he said.

"It's a January to forget," Mr McIntosh said. "I wouldn't be surprised if, by the end of the month, that 2019 was the warmest January on record."

The Bureau of Meteorology also expects records to be challenged, particularly for minimums.

The next couple of days will see the mercury in the city only drop to 23 degrees before the arrival of a cool change brings some relief by Friday morning.

Western suburbs have been doing tough. Penrith, for instance, is forecast to round out January with 38 and 41-degree maximums after topping 37 degrees on Tuesday.

Mohammed Nabi, a senior forecaster at the bureau, said steady onshore winds, particularly from the north-east, have lifted humidity levels along the coast.

"You're looking at a fairly uncomfortable summer," Mr Nabi said. "It's been tropical at times."

January is Sydney's hottest month in a typical year, although humidity tends to be higher in February - a pattern residents will probably hope doesn't repeat in 2019.

This summer's weather has been dominated by "an ever-present high pressure system that has parked itself over the Tasman", Mr Nabi said. "It's not moved much over the whole summer."

Any cold fronts have struggled to make it to Sydney and have done little to break up the inland heat.

A series of heatwaves has smashed records at many sites as well as breaking the highest minimum temperatures for Australia on a couple of occasions.

The bureau and the CSIRO noted in the State of the Climate 2018 report that temperatures across Australia have warmed about 1 degree over the past century with climate change, leading to an increase in the frequency of extreme heat events.

Warm January follows record hot end to 2018

A hot January across much of Australia followed a record warm December for maximum, minimum and mean temperatures for the country.

The maximum anomaly for last month was 2.41 degrees, area-averaged for Australia, eclipsing the 2.3-degree previous record set in 1972, the bureau said. The departure from the norm was even more significant for minimums, with the 1.85-degree anomaly beating the previous high, also in 1972, by 0.63 degrees.

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