Severe thunderstorms, rainfall up to 40mm per hour possible for Northland and Auckland

A severe thunderstorm watch is in place for Northland and Auckland with localised downpours possible.

The watch has been issued by MetService, lasting nine hours from noon until 9pm on Friday.

"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.

MetService said its next update about the watch would be issued at 11am.