A heavy rain watch with the threat of thunderstorms is in place for the Bay of Plenty.

A winter blast is hitting the country with temperatures forecast to drop this week and heavy rainfall forecasted for the upper and eastern areas of the North Island. Snow warnings are predicted for the South Island.

The watch for the Bay of Plenty - including Rotorua, Tauranga and Whakatāne - came into force at 3am and is in place until 9pm today.

Periods of heavy rain with thunderstorms were possible this morning. Amounts may approach warning criteria, MetService said.

A significant low is moving across central New Zealand today and tomorrow, with the Cook Strait ferry services cancelled due to monster 7-metre swells.

Auckland and Northland expect rain and possible thunderstorms, while Gisborne faces heavy rain and potential flooding.