Temperatures in 2018 continued to hit record highs, with scientists calling the year a continuation of an "alarming trend".
The latest National Institute of Water and Atmospheric Research climate summary report released today showed the year kicked off with the hottest month ever recorded.
January's mean temperature last year was over 20.3C, which was 3.1C higher than the average.
The year as a whole was the second-equal warmest on record, along with 1998.
The average temperature was 13.41C, not quite reaching the high set in 2016 with an average of 13.45C.
NIWA principal scientist Chris Brandolino said four of the past six years were now in the top five of warmest ever recorded, which was extremely concerning.
"[The year of] 2016 was the warmest, 2017 was the 5th warmest. This year equal-second warmest and I think 2015 was the third warmest," Mr Brandolino said.
"So four out of the past six years we've finished top five and unfortunately part of a long-term and alarming trend."
The incredibly hot January meant the summer of 2017-2018 could claim to be New Zealand's hottest ever - a record previously set in 1934-35.
Mr Brandolino said there were 49 locations which reached record or near record temperatures around the country.
Why so warm in 2018? [1/3]— NIWA Weather (@NiwaWeather) January 7, 2019
Early in 2018, the Tasman Sea and New Zealand coastal waters recorded some of its warmest sea surface temperatures on record during a marine heatwave ️
This fuelled record-breaking warmth on land. pic.twitter.com/VkZsJxyQR0
In particular, Levin where the records date back to 1895, had its warmest year ever.
Mr Brandolino said the warm weather was due to three main components - sea surface temperatures, air flow from tropic and sub-tropic areas and an increase in greenhouse gasses.
"The increases in greenhouses that we continue to see is warming in the background," he said.
"In other words, we are seeing a long-term tailwind of temperatures. Our changing climate is acting as a long-term tailwind for high temperatures."
Rainfall was described by NIWA as "lumpy" across the year with lots of rain in February and less rain than expected for some places in June, August and September.
In Timaru, where records go back to 1881, the town experienced its fourth wettest year ever.
Alexandra had the highest temperature with 38.7C, while the sunniest region during 2018 was Nelson.
The wettest place was Milford Sound and the driest Clyde.
So how is 2019 looking?— NIWA Weather (@NiwaWeather) January 7, 2019
️ The year is expected to start out on a warmer than average note countrywide.
️ Rainfall looks mixed – near normal for many areas, but a bit drier north, wetter southwest. pic.twitter.com/rGkABY2P28