The quake triggered a tsunami threat for the Pacific country, warning waves as high as a metre could be headed for Fiji's coastlines. The alert was later withdrawn.
The capital Suva has two siren systems, but they were not sounded following the tsunami alert.
Emergency Services Minister Inia Seruiratu told the Fiji Broadcasting Corporation there was no need for the sirens.
The head of the United Nations Office for the Coordination of Humanitarian Affairs, Sune Gudnitz, told Pacific Beat he was concerned not everyone was aware of the evacuations.
"The main issue here is to make sure that the alarm systems work and that means that there has got to be sirens because sirens are accessible to everyone," he said.
Mr Gudnitz said authorities did engage the police to help get people out of the markets and common areas in downtown Suva.
Around 70 per cent of Fiji's population lives on the main island Viti Levu, many of whom live in smaller coastal towns.
The US Geological Survey issued their earthquake alert at 10:52am local time, shortly followed by a tsunami alert from the Pacific Tsunami Warning Centre at 10:57am local time.
The Fiji department of Mineral Resources sent an email, sighted by the ABC, to local media at 11:17am Suva time, nearly 25 minutes after the first tremor.
"Meanwhile, [if there are complications], it depends on whether you are on your phone, or on your computer or on the radio at that particular moment, so the sirens really have to work and that's going to get people off the ground very quickly," Mr Gudnitz said.
"I have not heard anyone telling me they head a siren yesterday, and we are based very close to the coastline.
"I'm sure the Fijian Government is very aware of this, and it has also been mentioned quite a bit on social media that nobody heard any sirens, so I have a lot of faith that they will check the sirens and make sure that they work.
"Because it's in everybody's interests and it's critical."
Lands and Mineral Resources permanent secretary Malakai Finau told the Fiji Times his department was aware of room for improvement, and that communications channels got jammed in the minutes after the quake.
The ABC made repeated attempts to contact the Fiji National Disaster Management Office for comment.