The quake, which struck 150 km (90 miles) southwest of the town of Arawa, was relatively shallow, at 10 km (six miles). There were no immediate reports of damage or casualties.
It is the fourth quake to strike in the area over the past week. The Pacific Tsunami Warning Center issued a tsunami threat but this has been lifted.
Chris McKee, assistant director of the Geophysical Observatory in the capital, Port Moresby, said: "It's not a particularly strong earthquake in global terms, but it could still have generated a tsunami, so it's a bit too uncertain at this stage to be sure."
Papua New Guinea sits on the Ring of Fire, the arc of seismic faults around the Pacific where earthquakes are common, SkyNews reported.