Thunderstorms are set to lash the upper North Island this afternoon, bringing with it a risk of small tornadoes and torrential downpours.

MetService said heavy showers and electrical storms were expected to move from Waitomo north, including Auckland, with the thunder likely to see potentially damaging 100km/h gales and intense heavy rain.

Meanwhile, cooler temperatures have brought much-wanted relief from the hot and muggy weather conditions felt by many around the country in the last few days and weeks.

The South Island is set to see “unseasonable” snow around Fiordland, Southland, southern Otago, and the Canterbury Plains and Marlborough.

Sunny skies and high temperatures are expected to return by Thursday.

Heavy showers have started to fall in Auckland, with a forecast high of 26C and an overnight low of 16C.

Meterologist Dom Barry said an active cold front that is followed by cool southwesterlies will affect the country today and tomorrow - bringing rain or showers to most areas.

That will also sweep out the muggy and sticky air much of the country has experienced in recent weeks.

Most of the North Island is forecast to get showers or some rain at some point today - from Kaitāia to Whāngārei, Hamilton, Tauranga, Rotorua, Gisborne, Taupō, Palmerston North, Masterton and Blenheim.

Temperature highs in the North Island range from 20C to 29C in Napier, which was also the hottest place in New Zealand by 1pm today.

“Temperatures tonight are set to plummet in many areas, with Alexandra forecast to drop to 3C and Queenstown and Wānaka to 4C,” Barry said.

“Hastings will see a drop of 20C - from 32C to just 12C tonight.”

The same front is expected to bring with it thunderstorms today. Areas that may be impacted are Northland through to Waitomo, as well as Taupō, Bay of Plenty, Hawke’s Bay, the Canterbury Plains and Marlborough.

The risk of thunderstorms reduces over the coming days, as all the hot and humid air is swept away and the welcome cooler temperatures dominate, the weather authority said.

‘Unseasonable snow’ a possibility

Barry said the change to cooler air also brings the possibility of some unseasonable snow falling over the highest parts of Fiordland, Southland, Southern Otago and the Canterbury High Country.

“However, with the recent warm weather we have had heating the ground, it is unlikely any snow will settle,” he said.

Temperature highs in southern parts of the country were mostly in the low teens; with 14C in Christchurch, 12C in Dunedin, 10C in Queenstown and the same in Invercargill.

Those in Stewart Island have a wintry 8C and an overnight low of 4C which will mean that the thicker blankets will need to be brought out tonight.

On Wednesday, weather forecasters say a ridge of high pressure will approach the country - bringing mainly fine conditions for most again; except in the eastern coasts, where showers are expected.

That high ridge pressure will spread over Aotearoa by Thursday and will mean for sunny skies and settled weather for most heading into the weekend.

“With this, temperatures are once again set to rise into the mid-to-late 20s for most.”

On Friday, the frontal system is set to erode the high ridge and moves up towards Westland - bringing rain with possibly heavy falls to that region and a glimpse of what might be in store for the rest of the country at the weekend.