The quake had a depth of 35 kilometres, hitting at about 4am local time, according to the USGS.
The quake struck in the heart of mainland PNG in Southern Highlands province.
According to the Pacific Tsunami Warning Center, the quake did not present a regional tsunami threat.
About a half an hour later, a second earthquake with a magnitude of 5.5 struck in the same area. It had a depth of 39 kilometres.
PNG rests on the 'Ring of Fire', a known hotbed of seismic activity due to a tectonic plate that spans the Pacific.