Floating volcanic rock from an unusual undersea volcano has inundated coastal villages in Papua New Guinea, prompting food, water and health concerns in vulnerable communities.

Masses of thick pumice stone have washed ashore in Manus Province, trapping seafaring villagers.

It comes after several eruptions from an active undersea volcano in the Bismarck Sea, about 125 kilometres south-east of Manus.

Councillor Raymond Simeku from Baon village, one of the worst-impacted areas, said he had never seen anything like it.

“I was so shocked,” Mr Simeku said.

“We are stuck in the village, unable to travel out to market or go out to fish right now … and it’s continuing to build up.

“It’s sort of like a desert … you cannot come in and we cannot go out.”

Residents walk along the coastline that is inundated with floating volcanic pumice stone. (Supplied: Manus MP’s Office)

Pumice is a light-weight and highly porous form of rock created when lava from volcanic eruptions cools rapidly.

Dense and stagnant coverage in coastal areas can block sunlight and kill coral, fish and other marine animals.

Cr Simeku said people were extremely anxious, with food and water availability an immediate concern.

Residents of Baon rely heavily on boats for fishing and trading crops but have been unable to travel off their remote island for days.

“We tried one boat going out but it nearly sank like quicksand,” he said.

Residents are calling on the government to address the issue. (Supplied: Manus MP’s Office)

He said there was also a strain on water supplies, with people using drinking water to bathe because they could not wash in the sea.

“Our water supplies are actually running down because so much water has been consumed just for bathing,” he said.

“We need more water tanks to accommodate for water.”

He said access to medical services was also a concern, with villagers usually relying on boats to access health clinics.

Communities are concerned about their ability to travel and the impact on the environment. (Supplied: Manus MP’s Office)

Papua New Guinea Prime Minister James Marape said the government was conducting urgent assessments of the situation.

“We understand that discharges from this undersea volcanic activity are affecting movement, traffic, and creating uncertainty amongst our people,” he said in a statement.

“Our immediate concern is the safety and wellbeing of island communities in and around Manus, particularly.”

Mr Marape said the government was determining how best to respond.

“We want a total assessment of what is happening and how government can best prepare our people for the worst-case scenario,” he said.

The volcano has erupted sporadically since early May. (Facebook: Stanis Yohang)

The Titan Ridge volcano began erupting in early May, with intense explosions of ash visible from space and witnessed by some fishermen.

Cr Simeku said it could also be heard from Baon village.

“Some residents actually hear and see sort of lights and hear noise of eruptions sometimes during the night,” he said.

Steven Saunders from the Rabaul Volcanic Observatory said the event could happen once or twice in a century.

“It is still active, but much reduced. But being a natural phenomenon, it may pick up again,” he said.

“The pumice production has reduced significantly, but it is now being transported by wind and current. 

“They will determine if it builds up on islands or gets dispersed.”