A climate scientist says last month was New Zealand's warmest July on the books, going by a measure that took in more than the official number of stations.

Niwa is shortly due to release the official statistics for July, which its meteorologist Ben Noll last week said was on track to finish somewhere in the top five.

New Zealand's warmest July ever recorded, at 1.8C above average, came in 1998 – just as that year's devastating El Nino climate system was dissipating.

Toward the end of last month, the official July temperature record was tracking at about 1.5C above average.

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Niwa used the benchmark, long-running seven station series – but Professor Jim Salinger, who helped create that programme, has looked at the picture using 22 land stations.

By that wider measure, July's land temperatures had finished up a record 1.79C above average, while the Tasman Sea had come in at 0.72C.

Using the 22 stations measure, the July 1998 measurement comes in at second at 1.74C above average.

Salinger put much of it down to a Southern Annular Mode (SAM) - effectively a ring of climate variability that encircled the South Pole and extended out to the latitudes of New Zealand - in the positive phase.

In its positive phase, the SAM was associated with relatively light winds and more settled weather over New Zealand latitudes, together with enhanced westerly winds over the southern oceans.

Salinger also singled out warmer sea surfaces, the absence of any strong El Nino or La Nina influence, and a background of global warming.

Worldwide, some agencies were reporting July as the hottest month ever recorded, matching or even beating the previous record-holder, July 2016.

New Zealand's warm July, which has given way to a series of icy blasts more characteristic of winter, also marked the 30th straight month of above-average temperatures.

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The 30-month run, in which each month had finished above respective mean temperatures for the 1981-2010 period, included some of the most dramatic climate events ever observed in New Zealand.

Among them: our hottest summer (2017-18), our second hottest year (2018), our hottest month (January 2018) and two marine heatwaves – one which would likely be considered freakish even amid anticipated 2050 conditions.

The combined effect could be seen in the Southern Alps' snow-starved glaciers, which one scientist recently described as "sad and dirty" after another major melt.