There are only two runways in the vast Sepik River region of Papua New Guinea, and the Samaritan Aviation planes don't need either of them.

The two Cessna 206 float planes can land on the many waterways of the Sepik floodplain, allowing Samaritan Aviation to provide a free air ambulance service for the people living along PNG's longest river and its many tributaries.

Californian aircraft engineer Mark Palm runs Samaritan Aviation from the East Sepik capital, Wewak.

"In this area there's a river, the Sepik River, that's 700 miles long, 1,100 kilometres, there's one hospital in this area for 500,000 people," he said.

"We go to places that take three to five days for them to come in to the one hospital and we're able to go out there in a 45-minute flight, pick up a patient and bring them straight to the hospital.

"So this is basically a flying ambulance."

A Samaritan Aviation plane at a floating village. ( Supplied: Mark Palm )

Sepik villagers normally have to travel in canoes or small motorboats for days to get lifesaving medical treatment.

"This airplane or the two airplanes we have now is really a lifeline and we offer access and hope to these people who have none," Mr Palm said.

The service has transported more than 600 critically ill patients since it began flying six years ago.

A patient arrives in Wewak en route to a hospital. ( Supplied: Mark Palm )

Medical director Chris Cook said he sees a broad range of cases.

"We do everything from trauma patients, spear wounds, knife wounds, gunshot wounds to maternal cases, newborn babies, you name it," he said.

Much of Samaritan Aviation's work is in the area of maternal and child health ( Supplied: Mark Palm )

Samaritan Aviation, known locally by the Pidgin name Saman Balus, also transports medical supplies and equipment to outlying villages.

Montford Mambare manages a health clinic in the village of Biwat.

"Saman Balus is giving good services to the people along the Sepik River," he said.

"Basically when they bring medicines we are happy about it."

A Cessna 206 delivering medicines in the Sepik River region. ( Supplied: Mark Palm )

Mark Palm founded Samaritan Aviation in 2000 with his friend Gary Bustin, after visiting PNG as a 19-year-old some years before.

It took the men 10 years to raise the money for the first plane, but now Mr Palm's entire family lives in the provincial capital Wewak and runs the service as a Christian outreach organisation.

"I've gotten duck eggs, crocodile meat and watermelons and chickens and different things and it's all just them being thankful with what they have and that's what it's all about," he said.

Mr Palm's wife Kirsten and their three children provide follow-up support for patients in the provincial hospital.

"It's just so rewarding" Mrs Palm said.

"There's nothing that can replace helping other people have a chance at life."

Samaritan Aviation gets its funding from the PNG Government, charities like Oxfam, and private donors in the United States.

It has already expanded its service by adding a second plane, and is now looking at servicing other remote parts of PNG.