Powerful waves are starting to hit the South Island and will spread up the country's entire western coastline by midnight.

The long period swells are so strong boaties are being warned against making any bar crossings and fishers told to stay off rocks or ledges for fear of being swept away.

The swells, which have started from a storm deep in the Southern Ocean, rapidly develop when they hit with powerful sets surging on to beaches. The 15-second intervals between waves means they are so packed with energy and water they create dangerous conditions near beaches, rock ledges and reefs.

MetService forecaster April Clark said the waves began hitting the South Island this morning and would be hitting Whanganui and Taranaki coastline this afternoon, reaching Northland during the evening.

Advertisement
Advertise with NZME.

"So they're spared for the afternoon but it'll be early evening that those long period waves will be moving right over that upper North Island west coast," she said.

Clark said the waves were expected to ease off tomorrow.

She said beachgoers and fishers needed to be aware the sea was not behaving as it normally did. The ferocious waves had the potential to catch people off guard by the long intervals between peaks breaking on the shore.

"You'll be in the lull of that wave for quite a while then all of a sudden you have a creeping wave that gets bigger and bigger," said Clark.