UPDATED 9.53AM: The MetService has further extended is severe weather watch to encompass the Bay of Plenty.

Heavy rain is expected in Auckland, Coromandel Peninsula, Waikato, Waitomo and now the Western Bay of Plenty through today and tomorrow.

"A deepening trough lies just west of Northland and over the next couple of days is expected to slowly extend southwards," says a statement from the weather organisation.

"Areas of rain associated with the trough should also gradually spread southwards with localised heavy falls."

EARLIER:

A severe weather watch for the Coromandel has now been extended to cover Great Barrier Island, Taumaranui and Taupo.

Heavy rain with the chance of thunderstorms is possible for these regions.

The MetService is also forecasting rain and low cloud for the Bay of Plenty today.

Rain is expected to continue in the Bay tomorrow, with heavy rain a possibility.

'A deepening trough currently lies just west of Northland and over the next couple of days is expected to slowly extend southwards,” says the MetService.

'Areas of rain associated with the trough should also gradually spread southwards with some localised heavy falls.

'The heaviest rain is currently expected over the water west of the North Island, but this watch is for the possibllty that rainfall over the above areas may reach warning criteria.”