There are still a couple of days left in the season, but prolonged warm weather and less rain than normal means Niwa scientists believe Dunedin, Invercargill and Milford Sound are on track to post their warmest autumns on record.
However, rain this week could see rankings change, climate scientist Nava Fedaeff said.
Potential autumn record breakers were Milford Sound (records began in 1934), Mt Cook (records began 1929), Tekapo (records began 1927), Dunedin (records began 1947) and Invercargill (records began 1905).
Gore is on track for its second-warmest autumn since records began in 1907, and Hokitika is on track for its fourth-warmest autumn since records began in 1866.
Ms Fedaeff said dry conditions were also a key feature of the autumn summary, and Tiwai Point (Southland) was likely to break its record for the driest autumn.
"The warmth has largely been a result of a lack of southerlies, and frequent bouts of high pressure.
"The seas around New Zealand also remain warmer than average, which has also contributed to warmer temperatures.
"New Zealand has not experienced a nationwide monthly mean temperature that was below average (0.51degC to 1.20degC below the 1981-2010 average) in 28 months, or since January 2017.''
Niwa is expected to release its official autumn climate summary next week, and its winter climate outlook will be released later this week.
Autumn is expected come to a wet, windy and chilly end for the South.
An active front is forecast to approach from the southwest today and slowly move northeast over the south of the South Island tomorrow.
There is high confidence of warning amounts of rain in Fiordland today and tomorrow, and there are also warning amounts of rain in Westland and the headwaters of Otago from today until the early hours of Friday.
There is also moderate confidence of severe gale northwesterlies in exposed inland parts of Otago today and tomorrow.
