Summer has only just begun, but Whanganui's summer is already a record-breaker.

As of 9am Monday, Whanganui has experienced 40 dry days in a row - beating its previous record of 38 days. A dry day is one in which less than 1mm of rain has fallen in a 24-hour period.

Georgina Griffiths from the MetService said Whanganui's rain records at Spriggens Park go back to 1937.

A very small amount of rain fell on the city on Monday morning, but Ms Griffiths said it was not enough to register. MetService registers rainfall above 0.2mm.

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Whanganui hasn't registered any rain at all since November 26 - and that was just 0.8mm.

"It's a very significant - a record-breaking - dry spell," Ms Griffiths said.

She said the dry conditions were exacerbated by consistently warm weather, which had not been seen in previous dry spells.

"We're expecting December's mean temperatures to be record or near-record highs," Ms Griffiths said.

She said the weather was caused by unusually warm seas - at least 2C higher than normal for this time of year.

"We usually have winds out at sea to stir things up, but this year it's been very calm."

And is the warm, dry weather likely to continue?

Ms Griffiths said Whanganui can expect some cooler temperatures on Tuesday evening and into Wednesday. Then it's back to more warm, dry weather.

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For Christmas Day, MetService is forecasting a cloudy day, but no rain, with a high of 26C.

Whanganui's fire risk danger is currently at high - and edging towards very high - and fire restrictions have been introduced.

The Minister for Primary Industries, Damien O'Connor, has held off on declaring a drought in the lower North Island, saying the criteria for classifying the conditions as a medium or large scale adverse event had not yet been met.