January was officially the hottest month since records began in 1909.
NIWA figures show average temperatures for the month of January across the country was 20.3°C.
The temperature for January normally averages 17.1°.
NIWA climate scientist Gregor Macara said the month's temperatures were unprecedented.
"It was unusual that the entire country seemed to observe temperatures that weren't only above average, but really considerably above average."
"The majority of observation stations we had observed temperatures more than 3° above normal and in fact there are quite a few sites that were 4° above normal which were essentially unprecedented - particularly for this time of year," he said.
New Zealand experienced intense heat over the 31 days with the highest temperature recorded as 37.6° in the Central Otago town, Clyde.
Other hot spots included Otago, the West Coast, Kāpiti Coast, and Taranaki, which all observed temperatures four degrees higher than normal.
The extreme temperatures were caused by higher sea temperatures and northerly winds, Mr Macara said.
He said that as an island nation, New Zealand was heavily influenced by the sea's temperature.
The previous hottest recorded month was February 1998, when temperatures hit 19.6°.