An unseasonably cold outbreak is set to bring heavy rain, thunderstorms and gales to many parts of the country on Monday.

MetService says a trough will bring unsettled weather to western parts of both islands, followed by heavy showers and thunderstorms across southern and central New Zealand.

Orange heavy rain warnings are in place for Fiordland north of Doubtful Sound and for the Otago Headwaters, with peak rates of up to 25 mm/h expected in parts of both places.

Heavy showers and thunderstorms for Fiordland could continue into Monday afternoon, the forecaster says, with alpine snow likely.

An orange strong wind warning is also in place for the Canterbury High Country until 10am, with severe gale northwesterlies forecast to gust 120km/h in exposed places.

The ranges of the Westland District and the headwaters of Canterbury lakes and rivers from Arthurs Pass southwards are expected to see a period of heavy rain until noon, with Marlborough under a strong wind watch until the same time.

Monday's weather would bring "a bit of everything", MetService's John Law told RNZ's Morning Report, with "some really intense thunderstorms already showing up quite nicely on the charts" and southwesterly swells of between four and six metres expected on parts of the western coast between Taranaki and Wellington.

Central parts of the country and Canterbury can expect a particularly blustery start to the day, but the winds will also increase across the North Island as the day progresses, he says.

"I think we're all going to find a fairly wet story, especially across those more western sides of the country."

Weatherwatch's Philip Duncan told RNZ's First Up the strongest forecast winds have the potential to cause damage to trees and powerlines.

"We are like MetService, we're both sort of saying gusts to 120km/h, so once you get over gale force - which is half that, about 60-64km/h - that's when you start to get branches breaking. So when you're at 120km/h [it] just increases the size of those branches, and if they're anywhere near a powerline, then [that] leads to other problems as well."

In the North Island, there are orange heavy rain warnings in place for Mount Taranaki until 4pm and for the Tararua Range into this evening.

Surface flooding and slips are possible, MetService says.

Heavy rain was expected from mid-morning for Waitomo, Taumarunui, Taupō, Taranaki away from the mountain, Taihape, Whanganui and Manawatū, the ranges of eastern Bay of Plenty and Gisborne/Tai Rāwhiti, and Wellington and Horowhenua Kapiti Coast, away from the Tararua Range.

Northwest winds could also approach severe gale in exposed places across Waitomo, Taumarunui, Taupō, Taranaki, Taihape, Whanganui, Manawatū north of Feilding, inland Hawke's Bay, Wairarapa south of about Greytown and Wellington, the forecaster says.

The weather will drive temperatures down in many parts of the country, Duncan says, but the storm itself is likely to be short-lived.

"Most places have a temperature drop coming up ... the lower half of the South Island will be the ones to feel it the most, but even the top of the country will also notice a temperature drop," he says.

However, it's "just a two-day blip; it's gone by Wednesday, pretty much, but we're in for a bit of a wild ride in some areas today".

Law agrees there will be "a real chill in the air for Tuesday", with highs of just 12 degrees forecast for Queenstown and Dunedin and he urges people to keep an eye on overnight temperatures.

-RNZ.