A spokesman at Papua New Guinea's National Disaster Centre said by telephone the affected area was very remote and the agency could not properly assess damage until communication was re-established.
The US Geological Survey said the quake hit about 90 kilometres southwest of Porgera on Monday morning. "There seem to be quite a few reports of the quake being felt strongly", he said.
Despite the huge magnitude, officials said there was no tsunami risk.
USA seismologists said some casualties and damage were possible.
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Its depth was around 35 kilometres (22 miles), USGS said.
Many people in the region lead subsistence lifestyles and homes constructed with local materials would likely hold up fairly well, McKee said.
USGS said some casualties and damage were possible and recent earthquakes in the area had caused tsunamis and landslides. "ExxonMobil PNG has confirmed that the PNG LNG facility at Hides has also been safely shut down".
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.
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