At lunchtime lightning started cracking above Taihape and the Tararua and Hawke's Bay Ranges.
Successive storms were expected to roll across Taumarunui, Taupo, Taihape, Taranaki, Whanganui Hill Country, Manawatu, Hawke's Bay and Wairarapa until 8.30pm. Some downpours are expected to be so bad up to 40mm of rain could fall in an hour accompanied by large hailstones.
Motorists are being warned to take care driving through deluges with the threat of flash flooding, slips and slick conditions making roads hazardous.
Much of the South Island is under a thunderstorm watch with thunder and lightning expected to rumble across the skies this afternoon and tonight.
MetService said unstable conditions over inland parts of the central and lower North Island and South Island were generating thundery activity with scattered heavy showers and isolated thunderstorms likely in many places for the rest of the day.
Weatherwatch.co.nz said thunderstorms were kicking off as daytime heating and converging winds lead to significant inland downpours across New Zealand.
Thunder and lightning was already starting across the centre of the North Island and regions flanking the Tararua Ranges.
The rainstorms were expected to worsen throughout the day with thunderstorms expected to roll over the upper South Island ranges.
Those in the main risk zones were at risk of severe thunderstorms, gusty winds, hail and flash flooding.
