The Bay of Plenty and Coromandel is on thunderstorm watch as an active frontal system approaches the country.

The MetService says approaching system is expected to cross the country during Saturday.

A thunderstorm watch has been issued for the Bay of Plenty and Coromandel. File photo.

'There is an associated moderate risk of thunderstorms developing in Northland, northern Auckland and northwest Nelson this evening, with this risk then spreading to remaining areas of Auckland, Coromandel Peninsula, Waikato and Bay of Plenty overnight Friday and Saturday morning,” says a statement from the weather organisation.

'Additionally there is a moderate risk that some of these thunderstorms could become severe for a time producing localised downpours of 25 to 35mm/hr.”

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.