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Observatory Hill - where records date back 159 years - notched an average maximum of 24.9 degrees, eclipsing the previous autumn record of 24.8 degrees set in 2016, the bureau said.

The season compared with a typical autumn maximum of 22.2 degrees.

The Observatory Hill site also registered 49 days of at least 25 degrees, the most for any autumn, and more than double the long-run average of just 20 such days.

Nights at the location were about 1.8 degrees above the norm.

May was slightly less exceptionally warm for Sydney, coming in at 2.2 degrees above the average daily maximum.

Still, the eight May days of at least 25 degrees matched the record number set two years earlier.

Over the March-May period, Sydney Airport averaged 7.7 hours of sunshine a day, an hour above the norm.

Last month itself was even more unusually sunny, with 7.9 hours on average compared with 6.3 hours on a typical May day.

Drying out

The pattern of relatively large high-pressure systems that brought unusually warm conditions to Sydney and NSW also contributed to a relatively dry autumn for much of the nation.

Average rainfall for the season was one-third below normal. Last month, though, was the third-driest May on record with average rainfall of just 8.7 millimetres, just ahead of the record low 7.8 millimetres set in May 2008.

For NSW, autumn was the eighth driest, with rainfall 60 per cent below average. May's totals, though, were just one-quarter of the norm.

For the key Murray-Darling Basin food bowl, rainfall levels were down 60 per cent for autumn and more than two-thirds for May alone.

For Sydney's Observatory Hill, rainfall in autumn totalled 165 millimetres, or less than half the average. That made it the city's driest autumn since 2006, the bureau said.

In May alone, Sydney collected just 23.2 millimetres, or just one-fifth the total.

Sydney's dam levels are running at just over 70 per cent full.  The largest reservoir, Warragamba, is at 73 per cent capacity, down more than one-fifth from a year earlier, according to WaterNSW.

There is little sign of an early return to more typical rain.

According to the bureau's winter outlook released on Thursday, odds favour below-average rain for south-eastern Australia and milder than average temperatures.