A 7.7 magnitude earthquake has struck near New Caledonia, causing warnings of unpredictable surges for New Zealand's coastal areas.

The National Emergency Management Agency (NEMA) is warning of danger to swimmers, surfers, people fishing, small boats and anyone in or near the water close to some shores in the upper North Island.

The key coastal areas which are being warned are: Ahipara Bay to Bay of Islands, Great Barrier Island, and from Matata to Tolaga Bay. People in these areas are advised to move out of the water, off beaches and shore areas and away from harbours, rivers and estuaries.

The agency says that some coastal areas could experience strong and unusual currents, and unpredictable surges at the shore.

However, it states that there is no need to evacuate other areas unless directly advised by local civil defence authorities.

Initial tsunami activity, such as strong currents and surges, was expected to reach New Zealand in the areas around North Cape at about 4.20am on Thursday.

NEMA decides any official warning status for New Zealand.

According to Geoscience Australia, the earthquake had an epicentre 401km east of Tadine, New Caledonia. Seismic data indicated the undersea earthquake struck at a depth of 54km.

The US Tsunami Warning System says a tsunami watch is in effect for American Samoa and there is potential for tsunamis in other regions including Vanuatu, Fiji and New Zealand.