A severe thunderstorm watch was currently in place for southern Northland, Auckland, down to Waikato and would be in place until 8pm tonight, Mr Corbett said.
MetService forecaster Hordur Thordarson said thunderstorms were expected to develop in Auckland, Waikato and Waitomo this afternoon.
"There is a risk that some of these could be slow moving and accompanied by downpours, 20 to 35mm/hr and hail.
"Rainfall of this intensity can cause flash flooding, especially about low-lying areas such as streams, rivers or narrow valleys, and may also lead to slips.
"Driving conditions will also be hazardous with surface flooding and poor visibility in heavy rain."
WeatherWatch spokesman Philip Duncan said afternoon cloud build ups were producing heavy showers around the Auckland, South Waikato and Central Plateau regions today.
The heavy showers could produce heavy downpours and risk localised flash and surface flooding, he said.
Isolated thunderstorms were also possible, Mr Duncan said.
"At the moment the main area of attention in the Auckland region is mostly north west of the CBD towards Helensville and down towards the Waitakere Ranges.
"Pockets of big showers will continue to pop up this afternoon around northern NZ before easing this evening."
Tomorrow more heavy showers and thunderstorms are expected across northern NZ including Auckland region, he said.
Meanwhile Hokitika and Milford were "basking in 23 - 24 degrees and sunshine", Mr Corbett said.
