The quake rattled Tonga early Wednesday at the same time that emergency authorities on the country's islands were broadcasting simulated alerts as part of a Pacific-wide simulation to test a warning system that spans the world's largest ocean.
The quake, the latest in a string of aftershocks to hit the country since a powerful 7.8 quake on May 4, caused no damage or injury.
Tonga's National Disaster Office deputy director Mali'u Takai said people in the Ha'apai island group near the epicenter of Wednesday's quake felt the ground move as officials were broadcasting mock tsunami warnings, sparking a rash of phone calls to emergency services.
The National Disaster Office was "bombarded with questions" as the quake hit in the midst of the tsunami test, Takai said.
He added that authorities should have done more to make clear to residents that the warnings, broadcast on local radio stations, were part of a test.
"We should have warned the people about it, because some ... don't realize it's an exercise," he added.
Nobody was evacuated as part of the Tonga's nationwide tsunami warning test. The country was one of more than two dozen around the Pacific taking part in the Pacific-wide drill.
There were no immediate reports of villagers fleeing from their homes on the nation's many low-lying coral islets.