Monster and "potentially damaging" waves are set to slam the coasts of both islands as a wintry blast sweeps up the country during the week.

MetService is forecasting waves of up to seven metres in height will pummel both islands during a 48-hour cold snap due to hit on Wednesday.

Biting southerly gales are forecast to batter the east coast of both islands as well as the capital with the polar air plunging the country back into winter-like conditions. Most of the South Island can expect single digit highs on Thursday.

Duty forecaster Angus Hines said the strong, southerly change would generate huge swells of up to five metres, with driving wind causing individual waves to easily reach seven metres in height.


The tempestuous seas would lash both the eastern and western coasts of both islands coming behind a front starting on Wednesday morning.

He said it was unusual the east coast would experience such high seas.

"In the 48 hours from Wednesday pretty much all coastlines will get some waves with the Bay of Plenty being the only sheltered area," he said.

"Significant and potentially damaging swells are going to charge up both sides of the country, starting on Wednesday evening. This will be most noticeable along the east coast, where such swells are uncommon."

Snow was expected to fall to low levels around Southland, Otago, Canterbury and Marlborough. It is also expected to fall down to 500m across parts of the North Island.

"It's quite a strong, chilly southerly that moves up and it's not going to be pleasant."

Today decent-sized waves of up to five metres were expected to form off Westland.

Winds were expected to whip waves up to six metres, but this was not unusual for this part of the country, he said.