The quake hit some 96 kilometres (60 miles) southwest of Panguna at 2:19 am (1local time yesterday), according to the US Geological Survey.
The epicentre was 104 kilometres southwest of the larger town of Arawa also on Bougainville Island and USGS said it was 30 kilometres deep, which is considered shallow. The seismologists had earlier reported the quake's strength at 6.7 magnitude, but later revised the figure.
The Hawaii-based Pacific Tsunami Warning Center had no alerts on its website after the quake. Quakes are common on the island nation that lies on the 4,000-kilometre-long Pacific Australia plate, which forms part of the "Ring of Fire", a hotspot for seismic activity due to friction between tectonic plates.