It is time to rug up as the year's first "winter storm" nears, with half a metre of snow forecast in the South Island as well as thunderstorms and gale force winds in the North Island today.

MetService meteorologist Arno Dyason said the looming storm was a "perfect set-up" to produce snow, combining heavy rain with freezing Antarctic air.

"A big low is developing in the Tasman Sea ... as it moves north these really strong southerlies are dragging up a lot of cold air, almost from the Antarctic.

"A lot of the heavy rain will turn into snow in elevated areas, especially in the South Island.

Advertisement

"It will feel like a real winter storm - quite unusual for this time of year."

Before the storm arrived, there would be a few showers across most of the country.

"But it will be pretty reasonable compared to what is coming," Dyason said.

Meteorologist Tom Bell said the North Island would be experiencing cooler temperatures, strong winds and thunderstorms from today.

A thunderstorm watch was in place for Waikato, Waitomo and Taranaki until 3pm today, while Northland, Auckland, Coromandel Peninsula, Bay of Plenty, Rotorua and Taupo were on watch until 5pm with winds expecting to hit up to 110km/h.

"Wind gusts of this strength can cause some structural damage, including trees and power lines, and may make driving hazardous."

Strong winds were also predicted across the West Coast of the South Island in Buller and Westland.

Gales force winds were expected to affect the upper South Island and many parts of the North Island on Tuesday.

Advertisement

Meanwhile, in the South Island heavy snow was predicted with the potential to cause traffic delays.

"That is going to have a pretty potentially big [impact] on traffic movement in the South Island," Bell said.

Heavy snow warnings were in place for the Kaikoura Ranges and the Canterbury Plains and High Country.

"Snow is also likely about the higher roads of the North Island later on Tuesday," MetService said.

Forecasts further into the week showed the harsh conditions weakening by Friday.

The deep low over Bay of Plenty causing the foul weather in the north should move away to the east on Wednesday and the associated severe weather should clear during Wednesday morning.

Brief heavy rain is possible over western areas of the South Island on Thursday and there was low confidence that rainfall would approach warning levels over Fiordland and southern Westland.

"Southwesterlies ease during Friday as a ridge approaches New Zealand from the west then crosses the country on Saturday."

Today's Forecast:

• Whangarei: Heavy showers with thunderstorms in morning. Strong southwest evening. High 23C. Low 9C.

• Auckland: Showers heavy with thunderstorms in morning. Westerlies to gale in evening. High 21C. Low 9C.

• Hamilton: Heavy rain and thunder turn to showers afternoon. High 20C. Low 5C.

• Tauranga: Showers and thunderstorms. High 21C. Low 6C.

• New Plymouth: Rain with thunderstorms. Gale winds in afternoon. High 20C. Low 5C.

• Napier: Showers. Northwesterlies in afternoon. High 23C Low 5C.

• Wellington: Rain heavy. Southerlies. High 15C Low 6C.

• Nelson: Rain easing in afternoon. Southeasterlies. High 17C. Low 4C.

• Christchurch: Rain to sleet in afternoon. Southwesterlies. High 9C. Low 4C.

• Dunedin: Sleety showers and strong southwesterlies. High 9C. Low 6C.