The offshore quake was just one kilometre deep (around half a mile) and centred 96 kilometres southwest of the town of Panguna.
Quakes of such magnitude are common in PNG, which sits on the so-called Pacific "Ring of Fire", a hotspot for seismic activity due to friction between tectonic plates.
"It's quite normal for that area," duty seismologist at Geoscience Australia Hugh Glanville told AFP of the quake.
"There would have been some shaking on the island but we don't expect much damage."