By Agence France-Presse
The quake's epicentre was around 90 kilometres (55 miles) south of Porgera in Enga province, according to the US Geological Survey, and hit at a depth of 35 kilometres.
The quake hit PNG's mountainous interior and US scientists said there was no tsunami threat.
Felix Taranu, a seismologist at the Geophysical Observatory in the capital Port Moresby said the quake was felt strongly at Mount Hagen, some 168 kilometres away.
He said social media posts reported blackouts and damage to buildings at Porgera, although he was not aware of any injuries.
"It will take some time to get assessment teams out there and get a clear picture of what's happening," he said.
The quake's epicentre is in rugged terrain about 612 kilometres north-west of Port Moresby.
USGS said some casualties and damage were possible and recent earthquakes in the area had caused landslides.
It said homes in the region were "a mix of vulnerable and earthquake resistant construction".
The impact should be relatively localised, it added.
Earthquakes 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.