New Zealand is less than three weeks away from recording its hottest year ever.
According to NIWA, 2016 is likely to break the previous record for warmth, set in 1998, and it is the stark reality of global warming.
National temperature records date back to 1909. So far this century just four years have been cooler than average temperatures.
The mean temperature for January to November this year is running at 0.94degC above the 1981-2010 average, NIWA says.
The World Meteorological Organisation says 2016 is set to be Earth's hottest year on record by a significant margin with temperatures 1.2degC above pre-industrial times.
A new high will be set for the planet for the third year in a row.
Seventy New Zealand centres are poised to set new mean temperature records. Auckland and Hamilton will most likely record their warmest years and Wellington its second warmest.
"What we are seeing is the stark reality of global warming," said NIWA principal climate scientist Brett Mullan.
Exceptionally warm conditions for the first six months of this year were caused by the long-term regional warming trend due to greenhouse gases, plus local factors.
"Sea surface temperatures in the Tasman were exceptionally warm and there was more northerly flow than usual over New Zealand, pushing local temperatures above the trend."
Carbon dioxide is the primary greenhouse gas contributing to atmospheric warming and levels have been steadily increasing for decades.
At NIWA's Baring Head Clean Air Station, near Wellington, the 400 parts per million (ppm) threshold was passed in June, about three years after the 400ppm threshold was first broken at Hawaii.
NIWA will confirm its final figures in a report due on January 9.