Holidaymakers along the entire west coast of the country are being warned large waves are due to roll in tomorrow, and they could catch people unaware.

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The MetService says there has been a belt of consistent gales off Australia for a few days, creating a swell that will reach New Zealand on New Year's Day.

Meterologist Tom Adams said it's expected waves won't be big first thing, but the swell will pick up quickly during the day.

"That's most dangerous because people will head out in the morning on their boats, or out for a swim and think the waves don't look too bad but then there's potential to be caught out, they might be trying to cross the bar on the way back in or something and with waves having picked up there's a risk obviously of things going wrong."

Tom Adams said the waves were expected to be three metres tomorrow, reducing to two metres on Wednesday.

He said sets of significantly bigger waves could also roll in, again, catching people out.

Mr Adams said any people hoping to get near the ocean over the next few days should be extra careful.

"When you've got large waves that does lead to increased rips... (or) you could be fishing on the rocks then a big wave could come through and catch you out."