The record temperature for New Zealand was measured in Timaru in South Canterbury in 2011, when it reached a scorching 41.3C.
The soaring temperatures come as both Niwa and climate scientist Dr Jim Salinger predicted 2018 to break, potentially, several records.
Preliminary data showed the country's average temperature for the first 23 days of the month was 19.85C - more than 2.72C higher than the current record.
Niwa meteorologist Chris Brandolino said if temperatures held for the remaining eight days, long-standing records were likely to be smashed.
Climate scientist Dr Jim Salinger was calling it a done deal, saying we are experiencing first-hand the hottest month in generations.
The current record warmest month dated back almost two decades to February 1998, with an average high of 19.61C.
Brandolino said there were three key elements contributing to the sweltering conditions.
The flow of air across the country in recent weeks and months had predominantly come from the warm northerly quarter.
This was coupled with a marine heatwave that had seen waters become much warmer around New Zealand, and climate change.
"When you throw these two things on top of a warming globe and warming atmosphere, that's when you get these record and near-record outcomes," said Brandalino.
