The tremor hit at a depth of 28 kilometres (17 miles) near sparsely populated New Britain island, 450 kilometres northeast of the capital Port Moresby, the United States Geological Survey said.
The Hawaii-based Pacific Tsunami Warning Center ruled out a widespread tsunami.
"Based on all available data, a destructive Pacific-wide tsunami is not expected," it said.
New Britain, the largest island of the Bismarck Archipelago, is east of mainland New Guinea and has a population of around 500,000 people.
It lies on the 4,000-kilometre Pacific-Australia plate, which forms part of the "Ring of Fire", a hotspot for seismic activity due to friction between tectonic plates.