Weather: Dangerous thunderstorms threaten North Island again

The severe thunderstorms that battered the North Island on Tuesday could be about to return.

Yesterday, the upper half of the island was hammered by large hail, lightning strikes, and heavy downpours.

Civil Defence warned the public to take shelter inside away from windows, and boaties were told to say off the water to avoid electrocution.

And the bad weather could be back, bringing fallen trees, crop damage and flash flooding to the wider region. The danger grows in the afternoon, with widespread thunder and lightning strikes as well as torrential rain and hail as large as 2cm.

"Cold air at upper levels combined with relatively warm sea surface temperatures is expected to bring unstable conditions to the upper North Island," MetService says.

"Wednesday afternoon and evening there is a moderate risk of thunderstorms in much of Northland, the far north of Auckland and in eastern Waikato and Waitomo as indicated on the chart. These storms may be accompanied by heavy rain.

"The storms in Northland and the far north of Auckland could be accompanied by 10 to 20mm hail and there is a low risk that these thunderstorms may become SEVERE during the afternoon with possible downpours and significant hail accumulations."

"Towering cumulus cloud well to the north of Auckland again this morning," a NIWA spokesperson said on Twitter.

"An early indicator that a few more thunderstorms are possible in the upper North Island today."

But there's good news for the wider area. No thunderstorms are expected elsewhere in New Zealand on Wednesday afternoon and evening, MetService says.