Cold mornings, thunderstorms and heavy rain across the west coast of the country are setting in this week.

Thunderstorms are forecast to bring bursts of heavy rain, hail and strong wind gusts of 90 to 100km/h from Taranaki to Northland, the far east of the Bay of Plenty, the far north of the Gisborne district, Buller and the far west of Nelson.

Coastal Southland, Fiordland and the south of Westland are expected to experience thunderstorms tonight, accompanied by hail and strong wind gusts from 90 to 100km/h.

The cold showery southwesterly flow will also bring heavy snow for parts of the lower South Island.

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Snow is expected to 200m across the lower South Island tonight overnight and early tomorrow morning, with some heavy falls above 400m.

There is also possibility of snow accumulations reaching warning criteria above 400m in Fiordland, Southland, Clutha and Otago.

MetService meteorologist Amy Rossiter said that snowfall levels could potentially lower to 100m in Southland tomorrow night and early on Wednesday.

"Further up the South Island we have got snow to about 500m around the Canterbury High Country and Arthur's Pass regions, to 600m around the Tararua Ranges and the lower North island, and down to 500m in the central plateau, Tongariro region.

"It will be a very cold start in the morning, with a lot of places seeing snow or sleety showers."

West to south-west winds battering Auckland will strengthen over the next 24 hours, peaking tomorrow afternoon, but easing by midnight.

Road Snowfall Warnings are in place for Desert Rd (SH1), Rimutaka Hill Rd (SH2), Lewis Pass (SH7), Arthur's Pass (SH73), Porters Pass (SH73), Haast Pass (SH6), Lindis Pass (SH8), Crown Range Rd, Milford Rd (SH94) and the Dunedin to Waitati Highway (SH1).

A slip has also closed the main highway between the Bay of Plenty and Gisborne.

The NZ Transport Agency said that the road will remain closed until further notice.

Contractors are on site clearing the slip, and geotechnical engineers are currently assessing the stability of the cliff face.

Motorists can use SH35 as an alternative route, and can check the NZTA website for updates.