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.