NIWA says 2018 was the second-equal hottest year on record, with new "extreme temperatures" around New Zealand.
It warns it's part of an "alarming trend" - and increases in greenhouse gases are to blame.
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The National Institute of Water and Atmospheric Research released its Annual Climate Summary on Tuesday morning, showing the average temperature hit 13.41degC - the highest average behind only 2016.
"Overall there were 73 high maximum and minimum temperature extremes, while only one low maximum extreme occurred," NIWA says.
This means annual average temperatures were as much as 1.2degC above the 1981-2010 average for the majority of New Zealand.
"A small strip of well above average (>1.20degC from average) temperatures were observed in southern Manawatu-Whanganui," NIWA says.
"Elsewhere, near average (within -0.50degC to +0.50degC of average) temperatures occurred in parts of southern Canterbury, Otago, small parts of Auckland and the Far North."
NIWA forecaster Chris Brandolino says it's part of a larger trend for New Zealand.
"What's really alarming is that four of the past six years have placed in the top five for mean warm temperatures in New Zealand," he told media on Tuesday.
2017's average temperature was 13.15degC - the fifth-warmest on record, behind 2016, 2013, 1999 and 1998 - which was the second-equal warmest.
Mr Brandolino says overall it was "a very warm year", with many places hitting new highs since records began in 1909.
"Six months we saw temperatures above average. Six months we saw temperatures near average and zero months we saw temperatures below average," he says.
"January was the warmest month of any month of any year on record with an alarming 3.1degC temperature anomaly."
He says the "record warmth" had three main components - sea surface temperatures, air flow from tropic and sub-tropic areas and an increase in greenhouse gasses.
"The increase in greenhouse gases that we continue to see is warming in the background," he says.
"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."
Parts of southern Canterbury had well above normal rainfall (over 149 percent of normal), while rainfall was also above normal across much of the eastern and upper South Island, as well as parts of Wellington, Wairarapa, Bay of Plenty, northern Waikato, and Auckland.
Rainfall was near normal for the rest of New Zealand.