MetService data shows it was the second-hottest September night since MetService records began in 1993.
At 3am yesterday, the temperature reached 21degC at Dunedin Airport and 22degC in Dunedin city.
Meteorologist Tom Adams said it narrowly missed equalling the September night record.
The warmest was on September 13, 2009, when the temperature reached 21.1degC at Dunedin Airport at 4am.
Mr Adams said the warm temperatures were caused by strong northwesterly gales and the ``flow-on heating'' effect, when air travelled up and over the Southern Alps, causing it to compress and heat up.
``That can happen day or night. It's not driven by the sun.''
Daytime temperatures yesterday were also hot, prompting many to head to the beach or sit in the sun.
The airport reached a high of 23degC at 2pm and Dunedin city reached an unofficial MetService temperature of 24degC.
However, neither broke any September records, he said.
Daytime-high records for the month were set in September 2006 and 2012, when 24.9degC was recorded at the airport.
He warned Dunedin residents to make the most of it.
The warm humid air mass that was sitting over much of Otago for the past two days was heading north and temperatures were expected to return to ``normal'' tomorrow, he said.
