The seismic movement was recorded 56km north-east of Ndoi Island, according to the US Geological Survey.
It is believed no tsunami alert has been issued.
The quake struck around 417km south-east of Suva, the capital.
A Geoscience Australia spokesman said quakes were common in the area.
"Earthquakes of this magnitude are not at all unusual in that part of the world, and it was very, very deep so it's something unlikely to have any impact on the surface," he said.