Parts of the North Island are in for a thundery night as a storm, which has already brought hail up to 25 millimetres in diameter passes through.
MetService has issued severe thunderstorm warnings for much of the upper North Island.
Forecaster Hordur Thordarson said there had been numerous reports of hail in the upper North Island, including stones which were 25mm in diameter around the Kaipara Harbour.
Stormy weather is expected over the next three days with small tornadoes also possible, but the weather was expected to improve by the weekend.
"Explosive'' rain clouds were pushing into the region ahead of a deep and large low in the southern Tasman Sea, said WeatherWatch.co.nz.
The website reported wild weather today including a waterspout - a funnel-shaped cloud that occurs over a body of water.
"About 11.15am decent-sized waterspout off Taranaki coast came to within about 500m of shoreline in Oakura.
Would be the biggest I have seen. Noticed many people standing at window in Oakura FourSquare watching it and taking shelter,'' a WeatherWatch reader reported.
The rough and unstable weather was to continue, said head weather analyst Philip Duncan.
"We have unpredictable weather that can produce rapidly forming large showers and thunderstorms, these can also produce tornadoes or waterspouts''.
He predicted a "high risk'' of tornado today but continued to stress the chance of one hitting property was still low.
Mr Duncan said if people saw a tornado, the safest place was indoors in a small room, away from windows.
Southerlies were expected from tomorrow, with snow likely to settle to about 300 metres in the South Island on Tuesday night and into Wednesday.
The Ministry of Civil Defence and Emergency Management advises that as storms approach you should:
* Take shelter, preferably indoors away from windows;
* Avoid sheltering under trees, if outside;
* Move cars under cover or away from trees;
* Secure any loose objects around your property;
* Check that drains and gutters are clear;
* Be ready to slow down or stop, if driving.
During and after the storm, you should also:
* Beware of fallen trees and power lines;
* Avoid streams and drains as you may be swept away in flash flooding.