Geoscience Australia says the quake, which struck early this morning, had an epicentre 400 kilometres east of the town of Tadine.
Seismic data indicates the undersea earthquake struck at a depth of 54 kilometres.
Several aftershocks of up to magnitude 6.1 have occurred.
The U.S. Tsunami Warning System says hazardous tsunami waves up to a level of 1 metre above the normal tide level are possible for coasts within 1-thousand kilometres of the epicentre with New Caledonia, Vanuatu particularly at risk.
Officials in American Samoa have cancelled a tsunami watch for the territory.
Our correspondent there said officials reported no significant wave was generated by the earthquake.
New Zealand's National Emergency Management Agency is warning that coastal areas there could experience strong and unusual currents, and unpredictable surges at the shore.
And Australia's Bureau of Meteorology said there is a tsunami threat to offshore Australian islands and territories.