The undersea earthquake struck south-east of the Loyalty Islands at 2.20am today, triggering the National Emergency Management Agency to issue an advisory for New Zealand.
NEMA said based on the most recent modelling and decreasing tsunami amplitudes at North Cape, Great Barrier Island and the East Cape, the tsunami threat has passed for New Zealand.
There could still be large unexpected currents and the public are advised to continue to take precautions in coastal zones for the rest of today, it said.
Early this morning, NEMA told people in or near the sea from Ahipara Bay to the Bay of Islands, Great Barrier Island, and from Matata to Tolaga Bay to move out of the water, off beaches and shore areas and away from harbours, rivers and estuaries.
However there was no need to evacuate other areas unless directly advised by local civil defence.
NEMA had said there was a danger to swimmers, surfers, people fishing, small boats and anyone in or near the water close to shore.
At Ahipara, on the southern end of Ninety Mile Beach on Northland's west coast, local man Paul Hansen said surge waves were pushing the water 30-40 metres further up the beach than usual.
Hansen, who lives above the beach, watched the surge come in from his house.
"I would expect the water to be sitting around about the mid-tide mark but on a reasonably regular basis what we're seeing is the water move right through to a high-tide mark.
"I will definitely be staying off the beach."
He said the beach entrances had been blocked off and he couldn't see anyone along the shoreline.
A water gauge off Great Barrier Island recorded a 75cm wave just before 6am.
NEMA had said the strong currents and surges would start around North Cape from about 4.20am, and the first activity might not be the most significant.
Acting director Roger Ball said there was no threat to land for New Zealand, but strong currents and surges were expected in some northern areas of the country.
"In those parts of New Zealand we are asking people to stay out of the water and stay off the beaches because of the possibility of surges and unusual currents.
"We don't expect any significant change to the current advisory unless there is a subsequent seismic event and we're constantly keeping an eye out for that but we don't have any information to indicate that at this time."
Meanwhile, Australia confirmed a marine tsunami threat to Lord Howe Island, a marine reserve more than 700km northeast of Sydney, but said evacuations were not necessary.
The quake, whose strength was upgraded from an earlier magnitude of 7.2, followed at least three other tremors in the region with magnitudes ranging from 5.7 to 6.1 in a span of just over an hour.
The US Tsunami Warning System said a tsunami watch was in effect for American Samoa and cited a potential for tsunamis in other regions including Vanuatu and Fiji.
Waves reaching up to a level of 1 metre above the normal tide level were possible for some coastal areas, it added.
Advice for people in areas under threat:
- Move out of the water, off beaches and shore areas and away from harbours, rivers and estuaries
- Do not go to the coast to watch the unusual wave activity as there may be dangerous and unpredictable surges
- Listen to local civil defence authorities and follow any instructions
- Share this information with family, neighbours and friends
GNS duty seismologist Dr John Ristau said there may be more earthquake activity in the South Pacific today, following the 7.7 quake.
"It's most likely that this 7.7 is going to be the main shock," he said. "However whenever you get an earthquake, particularly a large earthquake like this one, it does increase the risk for another large earthquake, even a bigger one, nearby.
"Even though that increased risk is still small, it's a possibility."
The subduction zone earthquake occurred at the boundary of the Australian plate as it pushes underneath the Pacific plate. "It wasn't close to land areas ... but it did generate a tsunami with a height of a few tens of centimetres," Ristau said.
- RNZ and Reuters
