By Agence France-Presse
Daily temperatures averaged 18.8 Celsius (65.84 Farenheit), 2.1C more than normal, the National Institute of Water and Atmospheric Research (NIWA) said.
With the mercury reaching as high as 38.7C in the South Island, NIWA said it was the hottest summer since records began in 1909, surpassing the previous high set in 1934-35.
NIWA's chief forecaster Chris Brandolino said a number of factors were behind the warm weather, including a spike in marine temperatures and warm northerly winds from a La Nina weather pattern.
Brandolino added that global warming due to climate change was also a major contributor because it lifted baseline temperatures over the long term.
He said more records were likely to fall in coming years if the factors behind man-made climate change were not addressed.
"The expectation is that as we work our way through the coming decades, or 10, 20, 30, 100 years from now, if we're on the same trajectory we are now with carbon emissions, this will continue," he told Radio New Zealand.
The high temperatures prompted the government to declare a mid-level drought in some areas of the country and provide assistance to struggling farmers.
Scientists have also linked extreme weather to climate change, including cyclone systems that have devastated Pacific island nations and caused flooding in New Zealand.
The country's fledgling centre-left government has vowed to help tackle climate change, with Prime Minister Jacinda Ardern describing it as the defining challenge of her generation.
She has set New Zealand a target of being carbon neutral by 2050.