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Summer temperatures have been 2.3°C hotter than average, 0.5° above the previous record, which dated back more than 80 years.
Niwa meteorologist Ben Noll said with summer not yet ended, it was likely the record-breaking average would push higher.
The big driver has been the marine heatwave - a signature of La Niña weather pattern.
"Our oceans around New Zealand have featured excessively warm conditions in the order of of two to four degrees above average and in some pockets six or seven degrees above average.
"As an island nation, as the seas go, we go."
These freakishly warmer seas, together with northerly winds from the tropics and sub-tropics, have ratcheted up the temperature and that was expected to continue through autumn, he said.
However, climate change was definitely a factor.
"If you had the same conditions, the same atmospheric set-up two or three decades ago, it's unlikely you would have experienced the anomalies or the difference from average being quite as extreme as it was in 2017/2018.
"And that's indicative of the long-term trend of global climate change, global warning."
New Zealand could expect to see increasing numbers of heatwaves and droughts in the coming decades, he said.
The country may continue to bask in warmer temperatures through autumn because because oceans cool more slowly than land.
"But this can also mean more fuel for weather systems [storms] tracking towards New Zealand from the tropics and sub-tropics....
"Sometimes it's too dry, sometimes it's too wet. This summer we've experienced both of those extremes."