The tremor struck at 3.49am at a depth of 608 kilometres.
It was centred 101 kilometres east-southeast of the capital Suva.
The Pacific Tsunami Warning Centre says the earthquake was so deep it is unlikely to make a dangerous tsunami.
There have been no reports of damage.
UPDATE - There is no tsunami threat to New Zealand following the 8.1M FIJI earthquake.— MCDEM (@NZcivildefence) September 6, 2018