Christchurch has been its second warmest on record at 18.8C. Its 19.7C record was set in 1998.
Forecasters say the scorching days and cloying mugginess could linger until at least mid next month but weather analysts are divided about what's causing it.
MetService says El Nino is responsible; Niwa and WeatherWatch argue bouts of rain this summer are out of character for an El Nino season.
MetService and WeatherWatch acknowledge that climate change could be playing a part, but a longer trend of rising temperatures will be needed to confirm that so, for now at least, super-hot summers are not "the new normal".
Meteorologist Georgina Griffiths of MetService said yesterday that with four days of the month to go, four of the six main centres are on track for their warmest February on record.
The previous hottest February in all but two of the major cities was in 1998, the last time there was a "super-strong El Nino".
The hot summer has been a mixed blessing for weather-dependent businesses. At one Auckland surf shop, business has boomed by "25 to 30 per cent" this summer.
Andre Newth, who owns Ultimate Surf and Skate Shop in Pinehill, says more people than usual have tried surfing this summer.
"It's an awesome summer. Wetsuit sales are down, surfboard sales are up," he joked.
Beginner and intermediate surfboards had been popular and rentals were doing well, too, especially on weekends, he said.
"We can sell six to 10 boards a day. We have a hire-board fleet, too, and our hire-board room on the weekends has been virtually empty. That translates into sales because people hire boards to try, and then they buy," Mr Newth said.
However, the summer hasn't been so kind to water companies.
Wellington tank-refill firm Bulk Water said its phones have barely rung in nearly a week.
Owner Glen Lovewell said commercial clients kept the business ticking over, but residential water tanks had been receiving regular top-ups from mother nature and had not needed his company's services.
"Because it's warmer, moisture forms from the Tasman and you get more rain; we get busy when the weather's cooler, believe it or not."
Ms Griffiths said it was normal for an El Nino summer to have such a high number of February days with temperatures topping 25C, but the slow creep of climate change was probably playing a part as well.
"Climate change is a long, slow escalator underlying everything. In the short term we have steps up and down -- cold seasons, warm ones, because of bog-standard weather," she explained.
"We got a step up because of the El Nino but there's probably an underlying climate change thing there too."
Ms Griffiths said high temperatures would linger until at least mid next month, when a warmer-than-usual autumn would begin.
The pattern of dry spells broken by rain was likely to continue.
Niwa scientist Chris Brandolino said preliminary predictions indicated the start of next month would be hot and sticky, with humidity in the North Island, and thunderstorms.
A brief cool-down would follow at the end of the first week for much of the country, followed by "settled weather and abnormally warm weather" in the second week.
WeatherWatch head analyst Philip Duncan agreed with Niwa, saying although El Nino could be responsible for hot, dry weather on New Zealand's east coasts, wet weather in other parts of the country was more like a La Nina year.
Meanwhile, Cyclone Winston could finally be petering out.
Winston dropped to Category 2 yesterday and WeatherWatch believes it will probably lose its cyclone status altogether at some point today or tomorrow.
The track to New Zealand is still unclear, as it has been for half a month now, but Mr Duncan said it was likely Winston would pull down tropical air over the country this weekend, which would merge with a cold front and bring heavy rain to some North Island areas.
