A severe thunderstorm watch has been issued for Auckland and Northland until 7pm - with warnings to be prepared for localised downpours and hail.
"A combination of unstable conditions and wind convergences over the upper North Island are leading to heavy shower and thunderstorm formation this afternoon and early evening," MetService says.
"There is a possibility that some thunderstorms may become SEVERE, bringing downpours of 25 to 35mm/h to Northland and Auckland, and these thunderstorms are likely to contain hail."
Already the storm has been blasting Auckland city, with weather cams showing it being lashed with rain and small hail.
The cause is a "complex low" in the Tasman Sea which is moving towards the top of the North Island.
Over Monday, the centre of the low tracks towards the Auckland/Northland area, dragging southerly winds over the rest of New Zealand.
MetService warns heavy showers and thunderstorms are a risk for both Auckland and Northland on Monday, while other parts of New Zealand will also get some showers.
However a ridge of high pressure moving onto the South Island on Monday should clear things up there.
