The first day of the new year kicks off on a bleak note for much of the country with cloud, showers and thunderstorms ushering in 2018.

You'll need to travel to eastern regions of both islands if you want to enjoy the summer sun. Hawke's Bay and Wairarapa, and Timaru south are the only parts expected to have clear skies across the day with nearly everywhere else shrouded in grey.

MetService forecaster Brian Mercer said rain would move into Taranaki and Waikato before spreading across much of the North Island.

A severe weather watch had been issued for Waitomo, Taumarunui and Taranaki with bursts of heavy rain and thunderstorms expected to drench the districts between 10am and 4pm.

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In Auckland showers were expected to become heavy after lunch.

Thunderstorms were expected to form over North Island inland ranges and Northland towards the end of the afternoon.

Mercer said the best place to find the sun would be Hawke's Bay and Wairarapa and the far south of the South Island.

"Places on the east coast up to Hawke's Bay will be nice this morning and should continue to be for most of the day."

Elsewhere it would be cloudy and wet as the frontal low hit the centre of the country.

He also warned people to be on the watch for dangerous, powerful waves expected to reach New Zealand today.

The exceptionally strong waves generated from a storm in the Southern Ocean would strike western beaches on both islands this evening posing a risk to boaties and those fishing off rocks and ledges.

Today's outlook for the main centres
Whangarei: Cloudy periods. Afternoon and evening showers, chance heavy. 26
Auckland: Cloudy, showers from afternoon, some heavy. 24
Hamilton: Cloudy, showers, fine afternoon spells. 24
Tauranga: Fine, afternoon or evening showers. 24
New Plymouth: Cloudy with a few showers, heavy this afternoon. 21
Wellington: Cloudy, showers then fine spells after lunch. 22
Nelson: Showers, becoming frequent in the afternoon. 22
Greymouth: Cloudy with occasional showers. 23
Christchurch: Morning showers then fine. 22
Dunedin: Fine apart from cloud morning and night. 19
Queenstown: Cloud clearing to fine. 25
Invercargill: Fine. 23

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