The PTWC said hazardous tsunami waves could hit coastal areas of Papua New Guinea, Indonesia, Nauru, and the Solomon Islands, while New Zealand's Ministry of Civil Defence issued a warning for all of the country's coast.
The earthquake struck 60 km to the east of Taron, New Ireland, at 8.51 pm local time (1051 GMT) at a depth of some 75 km, the USGS said.
The quake was originally recorded at 8.0 before being revised down to 7.9.
Quakes are common near Papua New Guinea, which lies on the 4,000-kilometre-long Pacific Australia plate, which forms part of the "Ring of Fire", a hotspot for seismic activity due to friction between tectonic plates.
In 2013 the neighbouring Solomon Islands were hit by a devastating tsunami after an 8.0-magnitude earthquake rattled the region. That tsunami left at least 10 people dead, destroyed hundreds of homes and left thousands of people homeless.
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