Wintry blast and severe weather hits the country

A frosty week ahead will plunge much of the South Island into single-digit temperatures with the first snowfall warning of 2018 predicted to affect many high-traffic Alpine passes.

In the North Island thunderstorms and gale-force winds are set to wreak havoc before the cooler air is felt.

A low pressure system in the Tasman Sea and a strong cold front from the Southern Ocean will bring severe weather to much of the country in the coming days.

Both Lewis Pass (State Highway 7) and Porters Pass (SH73) were expected to accumulate 40 to 50cm of snow at high levels with lesser amounts down to 400 metres on Monday overnight, according to MetService.

MetService meteorologist Claire Flynn said there were heavy snowfall warnings in place for the Canterbury High Country and Kaikoura Ranges above 400 metres, and snowfall watches for Otago, Southland and Fiordland above 300 metres.

"Those closer to the coast will likely get heavy rain," she said.

"While we may not see snow in Christchurch City, for example, higher roads of Banks Peninsula are likely to be affected by snowfall."

Snow was also likely for the central North Island from Tuesday night and into Wednesday as the arctic blast was set to gradually move up the South Island and into the North Island over the coming days, she said.

Christchurch was set to have a high of 8 or 9C on Tuesday, which considering it recently had a high of 27 was "a hell of a difference".

After two back-to-back nights predicted at a nippy 2 degrees, Queenstown was predicted to drop to -1 degree on Wednesday night.

Ashburton would be 1 degree on Wednesday night after two nights below 5 degrees.

MetService forecast heavy rain today for Fiordland and Westland ranges south of Otira.

The downpour was due to hit Buller, Nelson, Marlborough and Canterbury tomorrow.

While the deep south is feeling the cold, Auckland will get evening showers tomorrow, developing into heavy falls on Tuesday.

Gale-force winds were also predicted for the north within the next three days.

A southwesterly was rising to 15 knots on the Raglan coast this afternoon and was expected to rise to 35 knots by Monday evening.

Flynn said tying down trampolines, avoiding unnecessary travel, and clearing gutters were just a few steps people could take to prepare for the extreme weather ahead.

"There is the potential that the severe weather could disrupt power as well," she said.

"If people are unsure where to start, there is good information on preparing for severe weather at the Get Thru website.

"You can also check current road conditions and closures on the NZTA website."

Main centre Monday forecasts

Whangarei
A few showers, clearing evening. Westerlies. High 24C, low 16C.

Auckland
Fine breaks, but a few showers developing in the afternoon. Westerlies. High 23C, 16C.

Hamilton
A few showers. Westerlies. High 22C, 14C.

Tauranga
Fine breaks. A few showers developing in the afternoon. Westerlies. High 23C, 16C.

Wellington
A few showers, turning to rain at night. Southerlies turning northeast in the evening. High 17C, 9C.

Christchurch
Overcast with a few showers, then rain at night. Southeasterlies. High 13C, 5C.

Dunedin
Cloudy. A few showers from evening. Southwesterly picking up. High 11C, 5C.