A low pressure system over the Tasman Sea is expected to move slowly across the north of the North Island from Thursday night, through Friday and into Saturday, bringing periods of heavy rain.

"Localised downpours and thunderstorms are also possible. A Heavy Rain Watch is now in force for Northland, northern Auckland and the Coromandel Peninsula. Note, a Severe Thunderstorm Watch has also been issued for Northland and Great Barrier Island," says a MetService spokesperson.

The heavy rain watch issued in the Coromandel is valid from 4pm Friday, June 16, to 9am, Saturday June 17.

"Currently there is uncertainty with the distribution and intensity of the heaviest rain, and people are advised to keep up to date with the latest forecasts in case any areas in this Watch are upgraded to a Warning or new areas are added."

Areas under a heavy rain watch:

Area: Coromandel Peninsula
Valid: 17 hours from 4:00pm Fri 16 Jun to 9:00am Sat 17 Jun
Forecast: Periods of heavy rain. Rainfall amounts may approach warning criteria.

Area: Northland
Valid: 16 hours from 6:00pm Thu 15 Jun to 10:00am Fri 16 Jun
Forecast: Rain could be heavy at times. Rainfall amounts may approach warning criteria, especially in northern and eastern areas. Localised downpours of 25 to 40 mm/h and thunderstorms are possible.

Area: Auckland from Whangaparaoa northwards, also Great Barrier Island
Valid: 10 hours from 2:00am Fri 16 Jun to 12:00pm Fri 16 Jun
Forecast: Rain could be heavy at times. Localised downpours of 25 to 40 mm/h are possible.