The first month of this year is poised to be the warmest month New Zealand has ever had since records began in 1909.

Preliminary data is showing 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 is going one further, calling it a done deal and saying we are experiencing firsthand the hottest month in generations.

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The current record warmest month dated back almost two decades to February 1998 with an average high of 19.61C.

Another record that looked set to tumble was the difference from the average temperature, which dated back just after the Depression. That record was set in December 1934, when the difference from average was 2.67C.

"If things hold for January 2018 you would have the warmest temperature on record of any month and the largest difference from average ever," said Brandolino.

Brandolino said this record-to-be was coming hard on the heels of the second warmest December since records began and two of the warmest ever years on record.

He said there were three key elements contributing to the sweltering condition.

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.

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Salinger was confident the record would be broken and by an even more pronounced amount than the Niwa calculation.

"Putting in the temperature forecasts for the rest of the month brings the temperature up to 20.1C. It's a massive 3.1C above average and easily the warmest in records back to 1868," said Salinger.

"It's amazing," he said. "It's the warmest by half a degree."

He said the combination of global warming increasing temperatures by around a degree over the past century, the Southern Ocean in a positive annular mode keeping southern storms at bay, the marine heatwave and a weak La Nina pushing nor-easterly winds over the country had resulted in the hottest month on record.

Brandolino said the coming week inland areas of the South Island were set to swelter in the hottest temperatures of the year reaching to the upper 30s on Tuesday.

However, all eyes were on the tropics for the later half of the week, with a potentially devastating are of low pressure packed with rain and wind forming and moving on to New Zealand.

Brandolino said if it formed there was a chance the impact of the torrential weather combined with king tides would lead to similar flooding that inundated coastal regions at the start of the year.