The US Geological Survey said the quake occurred at a depth of 10 kilometres, about 163 kilometres east-northeast of Kandrian, New Britain, and some 576 kilometres from the capital Port Moresby.
There were no immediate reports of damage or injuries and the Pacific Tsunami Warning Center did not issue a tsunami alert.
Geoscience Australia, which also measured the tremor at 6.3-magnitude, said the quake was close to the coastline.
"People living in that local area would have gotten a fairly strong shake,'' seismologist David Jepsen said, adding that some vulnerable structures could have been damaged by the tremor.
"I don't think there would be a (local) tsunami,'' he added.
Mr Jepsen said quakes of such magnitude were common in the New Britain region of Papua New Guinea, which sits on the so-called Pacific Ring of Fire, a hotspot for seismic activity due to friction between tectonic plates.
A 6.7-magnitude jolt hit the country on Friday but there were no reports of damage in the impoverished Pacific island state.
A giant tsunami in 1998, caused by an undersea earthquake or a landslide, killed more than 2000 people near Aitape, on the country's northwest coast.