Tsunami waves were forecast to be less than 0.3 metres (one foot) high for the coasts of PNG and Solomon Islands, it said, adding that wave amplitudes could vary between regions.
A spokesman for PNG's National Disaster Management Office in Port Moresby said there were no immediate reports of damage from the quake but these typically took several hours to reach the capital after a major shake.
He was unable to comment on the tsunami threat.
The epicentre was 125 kilometres (78 miles) east of the town of Kimbe on the island of New Britain, according to the United States Geological Survey, which said it struck at a depth of around 40 km.
Two smaller tremors struck the region immediately before and after the main earthquake.
The USGS website said there was a "low likelihood of casualties and damage" from the quake.
But it warned that "recent earthquakes in this area have caused secondary hazards such as tsunamis, landslides and liquefaction that might have contributed to losses." PNG sits on the so-called Pacific Ring of Fire, a hotspot for seismic activity due to friction between tectonic plates.
The country is still recovering after a 7.5-magnitude quake hit its mountainous interior in February, killing at least 125 people, cutting off access to villages and knocking out power.
Its remote terrain means it often takes several days for information about damage from quakes to reach officials and aid agencies.
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