He said westerly winds circling the globe were trapped in a jet stream high in the atmosphere, meaning cooler polar air was unable to surge out and produce a good southerly.
As a result, all southerlies which were counted on to bring respite from the heat had not been "straight and true".
In a post on Facebook, MetService said rather than bringing cool, dry air off Antarctica, the winds had been tainted with warm air on the back of the sub-tropical lows hitting the country since November.
This was coupled with tropically fuelled northerly winds affecting the country over the past couple of months, resulting in the air over New Zealand getting warmer and gaining moisture.
It was not helped by sea-surface temperatures well above average, with air moving on to the eastern coasts of the country in southerly flows not cooled by the sea as much as they would have in previous years.
As a result, temperatures across the country had not fallen below 10C overnight for some time, said the MetService post.
And although it was normal for nights to be warm in summer, MetService said average overnight temperatures were warmer than average.
Auckland has stayed warmer than 15C since January 1 as the average overnight temperature hitting 18.6C.
Last night the city experienced its highest dew-point temperature of summer so far.
Niwa said the dew point, which is a measure of humidity, hit 22C, was because of the sub-tropical northerly air flow and warm seas. It's deemed uncomfortable when it goes higher than 18C.
Wellington too is fighting balmy conditions and average overnight lows of 16.1C, well above the usual 14.5C.
Even Christchurch has kept above 15C for the past six days, and an average minimum temperature about 2C above usual.
