A complex trough of low pressure is forecast to move across New Zealand from the west on Tuesday and Wednesday, says a MetService spokesperson.

"This will be preceded by a strong and moist northwest flow and followed by a cooler south to southwest flow.

"A ridge then builds over the country on Thursday. Another trough then moves across the far south from the southwest on Friday."

MetService is forecasting that on Tuesday and Wednesday, there is likely to be rainfall accumulations meeting warning criteria in Fiordland and Westland, while the risk of rainfall is low for Buller, northwest Tasman, the Tararua Range and Mount Taranaki.

In addition, northwest gales may become severe for inland parts of Southland and Otago, the Canterbury high country to Marlborough, Wellington, Wairarapa and the Tararua District on Tuesday and Wednesday.

Gale southwesterlies may become severe about coastal Otago on Wednesday and again on Friday.

Later on Wednesday, much colder southwesterlies could spread over the lower South Island, and MetService forecasters will monitor the possibility of unseasonal snowfall there.