MetService has issued a severe weather outlook for New Zealand.

"On Tuesday, a high pressure system over the Tasman Sea is expected to extend a ridge across New Zealand," says a MetService spokesperson.

"On Wednesday, a front from over the south Tasman Sea is expected to move onto the South Island, bringing strengthening west to northwest winds across the Island and rain with some heavy falls to Fiordland and Westland."

MetService says for Wednesday, there is moderate confidence that rainfall amounts will approach warning criteria in Fiordland and the southern half of Westland, and low confidence for warning amounts of rain for the northern half of Westland.

"There is also low confidence on Wednesday that west to northwest winds could reach severe gale in southern parts of Southland, including Stewart Island, and Clutha," says a MetService spokesperson.

The front is expected to weaken as it moves over central New Zealand during Thursday, while a cooler south to southwest flow spreads north over the South Island.

On Friday, a ridge of high pressure builds across the South Island, while a low pressure system to the north of the country extends an east to southeast flow over the north of the North Island