A powerful earthquake struck off the coast of Papua New Guinea on Saturday, generating a small tsunami and knocking out power in parts of the Pacific island nation. There were no immediate reports of injuries or damage.

The magnitude-7.9 quake struck 46 kilometres east of Taron in Papua New Guinea, the US Geological Survey said. The quake was deep, at 103 kilometres. Deeper earthquakes tend to cause less damage than shallow ones.

The USGS initially said the quake's magnitude was 8.0, but later downgraded the strength.

The Pacific Tsunami Warning Centre said there was a threat of a tsunami in Papua New Guinea and nearby areas. It said tsunami waves reaching 1-3 metres high were possible along the coasts of Papua New Guinea, while waves in other areas, including the Solomon Islands, would likely be less than 0.3 metre high.

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The New Zealand Ministry of Civil Defence & Emergency Management also issued an official tsunami warning for New Zealand, before later tweeting there was "no threat" to the country, and cancelling the warning.

A tsunami measuring less than 1 metre hit the coast of the island of New Ireland shortly after the earthquake, said Felix Taranu, seismologist with the Geophysical Observatory in the capital, Port Moresby. There were no immediate reports of damage from the tsunami or the quake, though officials were still working to contact people on the island, he said.

The quake knocked items off shelves and caused a blackout in the town of Kokopo in northeastern Papua New Guinea, Taranu said. But there were no reports of widespread damage in the town.

Papua New Guinea sits on the Ring of Fire, the arc of seismic faults around the Pacific Ocean where earthquakes are common.