Scientists have discovered two rare species of dolphins in a remote Papua New Guinea river system.

Researchers from James Cook University and the University of Papua New Guinea found Australian snubfin and humpback dolphins living in the Kikori Delta, in Gulf Province west of Port Moresby.

James Cook University senior research fellow Isabel Beasley said it was the only place outside of Australia that the rare snubfin dolphin has been seen.

"It seems like the Kikori Delta is really the only place in all of [Papua] New Guinea at this stage and — importantly — all of the Pacific Islands, where the snubfin dolphins are found," she said.

Researchers will now investigate whether the discovery shows marine mammals follow a similar separation to terrestrial species between Australasia and Asia — a delineation that runs between Lombok and Bali known as The Wallace Line.

"They seem to follow Wallace's Line broadly, but the next stage will be to see within the Indonesia archipelago and also into Papua and West Papua itself, where does the break lie and what's the driving force behind those species' little differences," Dr Beasley said.

Researchers say the small dolphin population needs to be protected. ( James Cook University: Isabel Beasley )

The International Union for the Conservation of Nature (IUCN) recognised the significance of the discovery by designating the river system as an Important Marine Mammal Area (IMMA).

It also designated parts of Bismark — an area now being targeted for experimental seabed mining — as an IMMA.

Dr Beasley said the Kikori River system now needed local protection and conservation.

"It was a great discovery in terms of actually finding the species," she said.

"It's really, really important now to work with government, NGOs, the local community as well to make sure we can conserve that very, very small, restricted population in the Kikori Delta."

The river system has now been designated as an Important Marine Mammal Area. ( James Cook University: Isabel Beasley )

The research was funded by Exxon-Mobil, which operates the PNG Liquefied Natural Gas project, part of which is in Gulf Province.

It included collaboration with PNG's Conservation and Environmental Protection Agency and National Museum and Art Gallery.

James Cook University said it would continue research in the Kikori Delta and expand its study to areas of the Bismark Sea with its PNG partners.