The Otago town recorded the highest national temperature for the month, 32.8degC, on Christmas Eve.
But it also observed the second-coldest national temperature, a frosty minus 2.1degC, on December 1.
''This is yet another example of the stark contrast between the first and second half of December,'' Niwa forecaster Chris Brandolino said.
Nationally in December, all the coolest temperatures occurred in the first 16 days, and the warmest in the last 15 days.
''The warmth was enough to not only make up for the temperature deficit but allow for a surplus,'' Mr Brandolino said.
Dunedin's mean temperature for the first half of the month was 11.9degC, 2degC cooler than normal.
By the end of the month, it was up to 14degC, 0.2degC above normal, Niwa said.
That was helped by 219 sunshine hours during the month, 131% of normal, making the city the sunniest of New Zealand's six main centres.
Queenstown experienced similar conditions, going from a mean temperature of 13.6degC at the start of the month to 15.3degC in the second half, 1.2degC above normal.
Dunedin was particularly cold on December 1.
That day it recorded only 10degC, its fourth-lowest maximum for December since records began in 1947, as well as its equal-third-lowest minimum of 4.4degC.
On the same day, Balclutha recorded its second-lowest maximum, 10.2degC.
The next day, Queenstown recorded its second-lowest minimum of the month, minus 0.2degC.
In contrast, Ranfurly recorded its fourth-highest December temperature on December 24 with 30degC and its third-highest minimum of 16.6degC on the 27th. But it also recorded its equal-third-lowest minimum on December 2, minus 1.2degC.
Cromwell and Lauder also recorded minimums on December 27 of 18.7degC and 19degC respectively.
Despite dry conditions in Otago, particularly in the north, no records for lack of rainfall were set.
The only place to come close was Balclutha, which recorded its third-lowest rainfall, 20mm, since records began in 1964.
''The continued lack of rainfall has led to severely to extremely dry soils for this time of year for much of eastern Canterbury and parts of Otago,'' Niwa said.
