It would have been felt by people 470 miles away and could have cause damage to buildings up to 37 miles away.
It is not known how much damage the earthquake has caused or if anyone has been injured.
No tsunami warning has been issued.
Seismologist Jonathan Bathgate, of Geoscience Australia, said: “It’s only 10 to 20 kilometres off the coast but still it’s very deep, so it wouldn’t be anything extreme at all. No tsunami.”
The earthquake was recorded at 1.22pm local time (4.22am GMT)on Wednesday.
Papua New Guinea has a population of around 7.3 million, and comprises of a cluster of islands in the southwestern part of the Pacific.
At the time, the Pacific Tsunami Warning Centre (PTWC) issued a warning for areas close to the quake, although there was no Pacific-wide threat of a tsunami, the Hawaii-based centre said.
The PTWC issued the preliminary warning of “hazardous tsunami waves” which covers the region of Vanuatu, Fiji and New Caledonia.