Heatwaves, storms, flooding: What's causing New Zealand's weather madness?

Wacky weather over the past few days has been caused by tropical air spreading over the country.

Hot, sunny and very humid conditions have covered many areas, particularly in the North Island, in the past few weeks.

But yesterday saw a change and slight reprieve from the heat -- with rain, at times heavy, falling over much of the North Island and flash flooding and thunderstorms in and around Auckland.

MetService severe weather forecaster Erick Brenstrum said it was not unusual to get thunderstorms in summer.

"What was going on with this event yesterday, we had very humid tropical air that had come down over New Zealand and that contained a lot more water. So that when you get showers and thunderstorms, the rain is much heavier," he said. "It was a bit like Auckland went to the tropics for a day."

Southwesterly conditions -- bringing colder winds -- are forecast to arrive at some point, but not just yet.

The remnants of ex-cyclone Victor was also another factor in the current tropical conditions, particularly the humidity.

"The low centre that has been left behind from the [former] tropical cyclone ... that is about 60km east of East Cape and the top of Gisborne province.

"It will be well clear of New Zealand by midnight tonight."

The heavy rain in Gisborne, which led to a severe weather warning for that region and east of Wairoa, is set to continue until later tonight.

"The rain is continuing in that area, but not quite as heavy as it was. That's the rain band that's around the ex-tropical cyclone," Mr Brenstrum said.

"But the fact that that has come close to New Zealand has been caused by there having been quite a bit of northeast winds over the ocean to the east of the North Island.

"And those northeasterlies have brought down tropical air -- as well as bringing down the remnants of the [ex]-tropical cyclone."