A large trough should lie over the country on Tuesday and Wednesday, bringing unsettled conditions and rain in the western Bay of Plenty and Coromandel Peninsula, says a MetService spokesperson.

"Heavy rain is possible in several places and there is already a Severe Weather Watch in place for the north.

"There continues to be a lot of variation in detail between various computer models, and people are advised to check the latest forecasts and warnings."

There is low confidence that warning amounts of rain will fall about the Coromandel Peninsula, Western Bay of Plenty, Mount Taranaki and the Tasman District on Tuesday and Wednesday and about eastern Bay of Plenty and northern Gisborne/Tairawhiti on Wednesday and Thursday.

For Westland and northern Fiordland there is moderate confidence that a rainfall warning will be needed on Wednesday and low confidence that warning amounts of rain will fall in the Grey and Buller Districts on Thursday.

"Gale north to northwest winds are possible with this trough and there is low confidence that severe gales will affect Wellington, Wairarapa, Marlborough and Canterbury on Wednesday and early Thursday.

"During Thursday, a southwest change should spread up the country as the trough moves off to the east and on Friday a high is expected to move over the North Island and northern South Island."