A powerful 7.6-magnitude undersea earthquake triggered tsunami alerts for New Zealand and Tonga Thursday but warnings were cancelled as the quake proved less destructive than feared.

The Pacific Tsunami Warning Center issued a tsunami alert after the quake struck off New Zealand's Kermadec Islands at 07:03 am Thursday (1903 GMT Wednesday) followed by a warning from local authorities.

The US Geological Survey initially measured the quake at magnitude 7.8 and at a depth of just one kilometre (half a mile) beneath the Pacific seabed, giving it the potential to cause destructive waves in Tonga and New Zealand.

But it was later revised to 7.6 and a depth of 20 kilometres.

The tsunami warning centre cancelled its warning at 1959 GMT and New Zealand Civil Defence followed suit about an hour later.

Civil Defence controller for Gisborne, John Davies, said the earthquake generated waves of up to one metre (60 centimetres) near the epicentre but there was no impact on the New Zealand coast.

"We haven't been able to observe anything, including (from) the tidal buoys that tell us whether a wave is arriving, so at this stage, no effect at all," he told Radio New Zealand.

He said a marine warning would remain in place for the rest of Thursday, with Civil Defence advising people not to venture out into coastal waters.

"(The quake) could cause undersea current changes and swells that could go on for the rest of the day but that is expected to be minimal and it's really a precautionary measure," he said.

No damage was reported in Tonga, where the publisher of the Matangi Tonga website Mary Lyn Fonua said some schools closed and people moved to higher ground as a precaution.

Civil Defence operations manager David Coetzee said New Zealand authorities always opted for a cautious approach and swiftly issued warnings following a quake of the magnitude that occurred off the Kermadecs.

"We can't wait until a proper, more confident assessment has been made," he told national radio.

"We need to act fast and in this case (it takes a tsunami) two to three hours' travel time from the Kermadecs to the first New Zealand coast. We take the precautionary route and do what we have to do."

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