UPDATED 6.30pm It's looking like a wet start to the long weekend for the upper North Island.

MetService has issued a severe weather watch for Northland, Auckland, Waikato, the Coromandel Peninsula and Bay of Plenty.

Heavy rain is forecast for today and tomorrow and up to 100 millimetres is forecast to fall in some places.

MetService Meteorologist Lisa Murray says Northland is experiencing the worst of it at the moment and it'll hit Auckland later on.

"Main rain band goes through overnight tonight and into tomorrow, and then it sort of lingers around, so there will be some showers that continue during the day as well."