The epicenter, about 71 miles underground, was about 54 miles south-southwest of Lae and 138 miles north-northwest of Port Moresby, the capital of Papua New Guinea.
No tsunami warning was issued.
Parts of the nation were without power, CNN reported.
"I was at my desk when it hit," Lae resident Ella Hall told CNN. "The house started to shake from side to side. I could hear glasses falling downstairs."
Police in Port Moresby reported some shaking, but said there were no immediate reports of damage.