Beachgoers are being urged to take care with strong swells expected to hit Whanganui beaches tomorrow morning.

Storm force winds south of Australia in the last week have generated exceptionally powerful waves which are heading to New Zealand.

These large long period waves will arrive on the southwest of the South Island Wednesday evening and then will sweep up the to the west coast of the North Island on Thursday.

While the swell is only expected to be four metres the swell period will be unusually long at 20 to 22 seconds, producing very large breaking waves inshore.

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"Sweeping onshore waves could be a danger to swimmers all along the west coast due to strong inshore currents, undertows and rips associated with this long period swell," MetService Meteorologist Lisa Murray said.

A wave period in excess of 20 seconds happens about once a year and could put boat moorings under stress or make harbour bars hazardous.

"Waves may look impressive but can be dangerous especially for those fishing on-shore or walking along coastal pathways", Ms Murray said.

The long period swells were expected to ease slowly over the next few days.

MetService advises people to keep up to date with our latest marine forecasts on the MetService website metservice.com.