At least 30 people died in Monday's magnitude-7.5 earthquake in Papua New Guinea, authorities have confirmed.

The Southern Highlands emergency coordinator said the deaths happened in the city of Mendi when the quake hit during the early hours of Monday morning.

Families were buried alive when hillsides collapsed and four were killed when a retaining wall fell on a house they were sleeping in.

Some residents were still trying to recover the bodies of family members buried in the landslides.

Authorities said they expected more deaths in outlying villages.

The PNG Government has struggled to assess the damage and casualties closest to the quake epicentre, because roads have been cut and bad weather has hampered access by helicopter.

Authorities are hoping to try again today to reach outlying districts and learn more about the impact of the quake.

The large earthquake damaged buildings and caused landslides across the highlands, and powerful aftershocks have continued to rattle the region.

The epicentre was about 96 kilometres south-west of Mendi, the capital of Southern Highlands province, a town of more than 50,000 people.