Thunder and lightning are rattling northern Auckland and the lower South Island as warm humid air moves over the country this weekend.
And NIWA says with the "unstable air mass in place" over New Zealand, additional thunderstorms are likely later on Saturday.
The cause is a tropical air flow coming down from the Pacific, causing a humid and rainy weekend.
"Sunday in particular looks wet, humid and mild with north to north-west winds and sub-tropical connections," said WeatherWatch head forecaster Philip Duncan.
"Regions like Waikato and Auckland will have humidity levels not dropping below the mid 80 percent range on Sunday and well into the 90 percent range overnight."
And Duncan said it's going to be warm as well as wet this weekend.
"Auckland has an overnight low of 17C on Saturday night and tonight it will only drop to 16C," he said.
"At the other end of New Zealand regions, like Southland, may reach daytime highs in the mid-20s with isolated thunderstorms on Sunday".
Thunderstorms have already been felt over Auckland and the lower South Island on Saturday afternoon, with multiple strikes recorded over the Auckland region.
And there's no let up forecast for Sunday. MetService expects a strong and moist northwest flow followed by a front to affect northern and western parts of the country before moving slowly to the east of East Cape on Monday.
"Heavy rain is forecast to affect northern and western regions, and strong or gale-force northwesterly winds are likely in exposed places. Severe weather warnings and watches have already been issued for some areas," it said.
"On Monday, a ridge of high pressure moves onto New Zealand from the west, bringing a brief period of settled weather.
"On Tuesday, another front, preceded by a strong northerly flow, approaches the South Island from the Tasman Sea.
"The front should move over southern New Zealand on Wednesday, followed by a disturbed northwesterly flow."
