The quake was 86km west south west of Port-Olry and was 10km below the earth's surface, the US Geological Survey (USGS) said. It struck just after 5am local time.
RNZ Pacific reports no tsunami warning was issued.
Cyclone Judy battered Vanuatu on Wednesday, cutting off telecommunication in parts of the nation, and just 24 hours later the country is preparing for a second cyclone.
A state of emergency was declared because of damage caused by the first storm.
