Big surf to hit local beaches

Beachgoers are being urged to take care with strong swells expected along the Whanganui coast today.

Storm-force winds south of Australia in the past week have generated long-period waves which were expected to arrive on the southwest of the South Island last night before sweeping up the west coast of the North Island today.

While the swell is only expected to be 4m, the swell period will be unusually long at 20 to 22 seconds, producing large waves inshore.

"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 over the next few days.

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