The water in the Mission Bay fountain has turned red, and no one knows why.

But the change in colour at the popular harbourside pool hasn't put off the hordes who flocked to the suburban Auckland attraction. Dozens frolicked in the fountain as temperatures in the city soared into the mid-20s this afternoon.

The cause of the discolouration remains a mystery - a council spokeswoman said this afternoon they were sending contractors to investigate.

The water at Mission Bay's fountain has turned red, but beach goers weren't bothered. (Photo / Dean Purcell)

It was definitely fountain-swimming weather in Auckland yesterday - Metservice confirmed the temperature in the city nudged 27C, continuing a warm summer's week in the city.

Masterton enjoyed the warmest temperatures, reaching 30C, followed by Alexandra and Lower Hutt on 29C.

The hot weather also sparked severe thunderstorms in the lower North Island, with 4370 strikes recorded between about noon and 7pm between Napier and Masterton and west to Wanganui - where 14.5mm an hour fell during the most intense period of the thunderstorm.

In Palmerston North downpours reached intensities of 13mm an hour.

Metservice meteorologist Sarah Haddon said the storm activity - which sparked more strikes than thunderstorms that rolled through the central North Island yesterday - were now easing as the temperature dropped.

They could return to the same areas tomorrow, but the risk was not as high as today.

Forecast heavy rain in the ranges behind Motueka, west of Nelson, also fell today. One station in the area recorded an hourly total of 27mm a hour.