The quake, which was initially reported at 6.9 magnitude before being revised lower, hit at a depth of 65 kilometres (40 miles) in the New Britain region.
"It would have been very widely felt," said seismologist Emma Mathews from Geoscience Australia, which put the quake's reading at 6.6 magnitude and a depth of 69 kilometres.
Mathews said while the threshold for a local tsunami was any quake within a depth of 100 kilometres, the Pacific Tsunami Warning Centre said that based on all the available data there was no threat of a destructive wave.
There were no immediate reports of damage.
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Earthquakes are common in Papua New Guinea, which sits on the so-called Pacific Ring of Fire, a hotspot for seismic activity due to friction between tectonic plates.
In April a powerful 7.1-magnitude tremor struck off Papua New Guinea off the town of Panguna on the remote and volcanic Bougainville island. It which was followed by a 6.7-magnitude quake a little further from the town.