"Based on the preliminary quake parameters" the Pacific Tsunami Warning Center said, "hazardous tsunami waves are possible for coasts located within 1000 kilometres of the natural disaster epicentre".
The undersea quake was only 10km deep and about 155km east-southeast of the Loyalty Islands off New Caledonia's east coast. Earthquakes are generally more destructive when the epicentre is near the surface.
Tsunami alerts have been issued for both New Caledonia and nearby Vanuatu, a state in the South Pacific consisting of some 80 islets with a population of 270,000 people.
People on the east coast have been told to retreat at least 300m from the shore by the New Caledonia civil defence, and try to get to ground of at least 12m in altitude.
Residents of New Caledonia, which lies north of New Zealand, received text messages directing them to go to refuges immediately, with waves of up to three metres possible, according to the warning centre, AFP reported.
The Pacific Tsunami Warning Centre said that tsunami waves had been "observed" in the region but did not say where.
Hazardous tsunami waves are forecast for some coastlines.
Authorities asked citizens to seek safety after the quake struck.
The New Zealand government's Ministry of Civil Defence and Emergency Management also said there is no tsunami risk to their country. USGS originally gave the magnitude of the quake as 7.6 before revising it downwards slightly. "The tsunami warning was what was different this time, though".