A 7.5-magnitude earthquake struck off the coast of Papua New Guinea Monday, US seismologists said, with "hazardous" tsunami waves possible in areas near the epicentre although no major damage was immediately reported.

The quake hit at a depth of 33 kilometres (20 miles), some 55 kilometres from the nearest city of Kokopo on New Britain island and 787 kilometres from the capital Port Moresby, the United States Geological Survey said.

"Based on all available data... hazardous tsunami waves are forecast for some coasts," the Pacific Tsunami Warning Centre said as a result of the quake, which was estimated earlier at 7.7 magnitude before being revised.

The tsunami risk was expected to be limited to PNG, with waves of between one and three metres possible.

Much smaller waves of less than 30 centimetres above the tide level could affect the coasts of other areas of the Pacific, including Australia, Japan, Philippines, New Caledonia, Marshall Islands, Fiji, Samoa, and Vanuatu, it added.

The initial quake was followed by a smaller 5.7-magnitude aftershock in the same area.

Geoscience Australia seismologist Jonathan Bathgate said there was a possibility of a local tsunami but based on the magnitude of the quake it was likely to be on the lower end of the scale.

"It would have been quite widely felt, certainly within 100 kilometres," he told AFP of the quake.

"It's probably not going to be damaging. Although people would have felt strong shaking... there's probably not going to be a lot of widely spread damage."

The nearest town of Kokopo, the capital of East New Britain, has a population of around 20,000. It started to grow after nearby Rabaul was flattened by volcanic eruptions in 1994.

- 'Everybody was afraid' -

Leonie Fakal, a housekeeper at the Seaview Beach Resort in Kokopo, said all the staff and their two guests ran outside when the quake hit.

"Everybody was a bit afraid. There was a lot of shaking and some things fell down in the hotel," she told AFP.

"Everyone ran outside but there was not much damage."

Chris McKee from the Geological Survey in Papua New Guinea said they were still analysing the quake.

"We've spoken with people in (the island of) New Ireland and (its major town of) Kavieng and they didn't even feel the earthquake which is a bit strange as it's been given as 7.7," he said, referring to the island neighbouring New Britain.

"Our system has given it as 7.4," he added, saying that the true magnitude was probably "somewhere in between".

"It's a big earthquake, fairly shallow. It's possible a tsunami was generated in the southern New Ireland area."

McKee added that constructions in the area were mostly wooden and "seem to be able to soak up the shakes quite well".

Earthquakes are common in PNG, which sits on the so-called Pacific Ring of Fire, a hotspot for seismic activity due to friction between tectonic plates.

The last big tremor off the Pacific nation's coast was a 6.8-magnitude quake that struck Bougainville island in December.

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.