The top of the North Island is in for 20 hours of rain and thunderstorms as tropically-fuelled disturbance sparks days of unsettled weather.

Heavy rain is forecast to plague the top half of the North Island and the north and west of the South Island from tomorrow evening and into Friday.

MetService meteorologist John Law said the areas most likely to see heavy rainfall would be the Bay of Plenty, Coromandel and the western side of the Tasman district.

A heavy rain watch has been issued for the Bay of Plenty and Rotorua from 10pm Thursday until 8pm on Friday and Taranaki from 10pm Thursday until 6pm Friday.

For the rest of the North Island, heavy rain watches would come into place for Northland from 1pm tomorrow and last for 22 hours though to Friday morning.

A heavy rain watch would also come into place for Auckland including Great Barrier Island and Coromandel Peninsula from 8pm tomorrow lasting 20 hours until 4pm Friday.

Thunderstorms with bursts of heavy rain are possible, MetService said.

For the South Island, a heavy rain watch would also come into force for Tasman from 6pm tomorrow until 6pm Friday while another is in place for the West Coast from 11am tomorrow until 3am Friday.

The National Institute of Water and Atmospheric Research (Niwa) said: “Two disturbances in the Tasman Sea will drag tropical moisture into New Zealand from Friday into next week.

“The combined system will produce rounds of heavy rainfall, particularly in the northern & eastern South Island, possibly swirling in the Tasman for up to a week.”

Law said the heavy rain and possible thunderstorms would also likely bring windy conditions.

“Especially in the overnight time, Thursday into Friday, into the northern areas,” he said.

“We are going to find a windy and wetter story overnight Thursday and into Friday.”

Law said the cause of this unsettled weather was a front coming from the west.

“As it runs towards us, an area of low pressure develops on that and you can imagine that with the low pressure, the winds wrap around it in a clockwise fashion,” he said.