Many places across New Zealand have enjoyed their warmest May on the books, as a climatic tussle continues across the equatorial Pacific.

But meteorologists say the balmier-than-usual month may be about to take a chilly turn, as a cold snap hits the country later this week.

According to running figures, Palmerston North, Turangi, Waiouru, Te Puke, Motu, Te Kuiti, Waipawa and Wairoa in the North Island and Farewell Spit, Westport, Culverden, Waiau, Cheviot, Medbury and Akaroa in the South Island have all seen their warmest May ever.

The main centres weren't too far behind, with Auckland (as measured at Mangere), Tauranga, Wellington (at the airport) had experienced their second-warmest May, as had Hamilton, where records stretched back to 1906.

The mean temperatures in Christchurch and Dunedin were both tracking at 1.2C above average, however that hadn't been high enough to place them within the top four warmest Mays.

"Over the first three weeks, a lot of towns have been tracking toward warmest May, but with a cold front moving up the western South Island and into the North Island, we will certainly feel the cold this weekend and early next week," Niwa meteorologist Ben Noll said.

"There will be some very chilly conditions - even for this time of year - with below average temperatures and much drier air, so it will be a good time to get the chapstick and the lotions out."
Noll expected the colder close to the month would throw May's finishing figures back toward average.

"It will probably knock the North Island closer to average and the South Island toward average or below average."

The background reasons for the warmer month weren't straightforward or as predictable as they had been under a long-persisting La Nina climate system that had now all but faded.

"Right now the equatorial Pacific is basically in transition - the La Nina is fading, and we're in something called ENSO-neutral," Noll said.

"During this, we see more variable conditions around the globe, New Zealand included, and it's almost like a see-saw, where the atmosphere is trying to go toward El Nino, but the ocean pulls it back in La Nina's direction.

"At its most basic point, El Nino tends to bring more south-westerlies to New Zealand, while La Nina tends to bring more northerlies and north-easterlies.

"But the interesting thing about this battle is that, as we go into June, it looks like La Nina-like conditions, or what we consider climate-forcing, may regain strength and try to come back for a time, which could lead to patterns that we saw earlier in the year."

That potentially included a fresh ridge of high pressure over the country, and northerlies and north-easterlies particularly over northern and eastern regions.

Niwa's outlook for the May-to-July period had forecast temperatures to be above average over Northland, Auckland, Waikato and the Bay of Plenty, and above average or average everywhere else.

The year had already seen the warmest first three months of any year on record, along with a warmest-ever January that marked the high-point of a record-hot summer.