Swells of up to 10m are expected in the Tasman Sea today and likely through the weekend.
The rough seas will be accompanied by heavy rain and strong wind, continuing a dramatic week of weather throughout the country.
Conditions in Cook Strait are being assessed as to whether ferry crossings can continue in the squally conditions.
WeatherWatch commentator Richard Green said he would certainly rather be on land today.
"These swells are particularly affecting western and southern parts of the country, but also some eastern areas, too.
"We've got these west-to-southwest swells as high as eight to 10 metres."
He said the swells were mostly due to a strong south-westerly airflow.
"A series of fronts embedded in the south-westerly air-flows tends to increase the winds and the swells.
"Not only the swells, but overhead condition are heavy and showery at times and have resulted in tornadoes in the north and also the black ice down south."
Mr Green said people should avoid being out on the water or fishing from rocks until things calmed down.
Meanwhile, wind gusts reached 115-kilometres an hour in Foveaux Strait this afternoon, as a strong, disturbed southwesterly hit the country.
Metservice has issued a strong wind watch for coastal Southland, Stewart Island and the Otago Peninsula through until tomorrow morning.
It said the strong winds would whip up large swells, which were already reaching eight metres in exposed places along the West Coast, and those big waves are expected to stick around throughout the weekend.
Earlier this week a tornado ripped through Northland, damaging 15 homes. There were hundreds of lightning strikes and torrential rain in Auckland, and cold weather in much of the rest of the country.