Tsunami warning sirens have sounded and thousands of New Zealanders on the North Island’s east coast have evacuated to higher ground after a third earthquake within hours on Friday.

There was gridlock in some cities and towns across Northland and the Bay of Plenty as workers, students and residents fled.

Civil defence officials were on the ground to help people evacuate as authorities said tsunami waves could reach three metres above tide levels.

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The deputy leader of the main opposition National Party, Dr Shane Reti, told RNZ he could see the sea going out in Whangarei, on the east coast of Northland.

There were also multiple reports of streams, rivers and the ocean emptying out in the region.

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By 9am (AEDT), the National Emergency Management Agency had warned residents to expect “coastal inundation” on the west coast of the North Island from Cape Reinga to Ahipara and on a much longer area of the island’s east coast, from the Cape to Whangarei and Matata to Tolaga Bay, in the Bay of Plenty.

Great Barrier Island is also included in the warnings.

“Strong and unusual currents and unpredictable surges near the shore are expected in the following areas,” AEMA said.

“This means a threat to beach, harbour, estuary and small boat activities.”

• See the latest NZ warnings here

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The first reported tidal surge rolled into Tokomaru Bay, on the east coast of the North Island, north of Gisborne, within hours.

There were also warnings of possible surges and strong currents for beaches on much of the North Island as far south as Wellington and the west coast near Palmerston North. Residents were urged to stay out of the water.

An emergency alert was issued for all coastal areas around Auckland, a city of 1.7 million.

“The first wave may not be the largest,” Bill Fry, a seismologist at geoscience body GNS said.

“Tsunami activity will continue for several hours and the threat must be regarded as real until this warning is cancelled,” he said.

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By 11.30am (AEDT), the warnings were being downgraded. The thousands of people who had been evacuated along massive stretches of the North Island coastline were told they could return home.

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Friday’s alarm came after three quakes near New Zealand and in the Pacific in the early hours of Friday.

The third had a magnitude of 8.1 and struck the Kermadec Islands, north-east of New Zealand’s North Island.

It came shortly after a 7.4 magnitude earthquake in the same region.

Earlier, a large 7.2 magnitude earthquake struck about 900 kilometres away on the east of the North Island. It was felt by more than 60,000 people across NZ, with many describing the shaking as “severe”.

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Aftershocks – including one at magnitude 6.2 – were still being recorded in the area later on Friday.

There were no reports of damage or casualties from the quakes.

Elsewhere, the Bureau of Meteorology confirmed a 64-centimetre wave had hit Norfolk Island on Friday morning.

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The bureau warned of the possibility of dangerous rips, waves and strong ocean currents around the island, as well as “some localised overflow into the immediate foreshore”.

“Norfolk Island residents are strongly advised to get out of the water and move away from the immediate water’s edge of beaches, marinas, coastal estuaries and rock platforms,” it said.

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Police have also warned residents not to stand on the coast to watch the tsunami, and boats around the island have been told to return to shore.

There is no tsunami threat to the Australian mainland.

Warnings were also issued for Pacific islands such as Tonga, American Samoa, Fiji, Vanuatu, Hawaii and others. Chile might also experience a minor tsunami.

-with AAP