Severe thunderstorms, rainfall up to 40mm per hour possible for top half of North Island

A severe thunderstorm watch is in place for Northland, Auckland, Waikato, Bay of Plenty and Taranaki with localised downpours possible.

MetService was forecasting a moderate risk of localised downpours with rainfall rates of between 25 and 40mm per hour from 11am.

The latest forecast warned that rainfall at that rate could cause flash flooding, especially in low-lying areas and could lead to slips.

"There is a moderate risk that thunderstorms will become severe during Friday afternoon and evening in southwestern parts of Northland (south of Kaitaia and Whangārei) and Auckland.

"Severe thunderstorms will produce localised downpours with rainfall rates of 25 to 40mm per hour."

Auckland Emergency Management took to social media to warn Aucklanders about the weather watch.

It said rainfall of up to 40mm per hour can cause surface and/or flash flooding - especially in low-lying areas such as streams, rivers or narrow valleys.

It could also lead to slips, it said.

In a statement, MetService meteorologist Alanna Burrows said there was also a risk of thunderstorms on Saturday - with parts of the South Island possibly in the firing line, too.

"With rain and the risk of thunderstorms ... it's worth keeping an eye on our rain radar and latest forecasts if you're heading out to the polling station vote," Burrows said.