The quake struck at 4am NZT, with its epicentre at a distance of 95km west-north-west of Hihifo, Tonga, the USGS said.
RNZ pacific reports it was at a depth of 210km.
The US Tsunami Warning System said there was no tsunami warning after the earthquake.
The quake was followed by a 5.1 magnitude aftershock at 4.30am NZT.
Hihifo is the main village on the island of Niuatoputapu north of the Tonga main island group.
In Samoa, people had evacuated to higher ground following the quake.
