A USGS map showed the tremor centred in a patch of ocean some 300 kilometres (186 miles) east of the town of Vanimo, which is preparing to host Pope Francis over the weekend.
A photographer based in the provincial capital of Wewak told AFP there did not appear to be major damage in the immediate aftermath of the quake.
Earthquakes are common in Papua New Guinea, which sits on top of the seismic "Ring of Fire" -- an arc of intense tectonic activity that stretches through Southeast Asia and across the Pacific basin.
Although they seldom cause widespread damage in sparsely populated areas, they can trigger destructive landslides.
Pope Francis arrived in Indonesia earlier this week on the first leg of his 12-day tour.
On Friday he heads to Christian-majority Papua New Guinea for a three-night visit that will see him briefly stop in Vanimo, a remote coastal town close to the country's border with Indonesia.
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