SYDNEY - A ferry with around 350 people on board sank off Papua New Guinea's north coast on Thursday, with Australian authorities warning of a high loss of life, although some survivors had been found already.

"This is obviously a major tragedy - 350 people on board a ship that has gone down," Australian Prime Minister Julia Gillard told reporters in Melbourne.

"It is likely a very high loss of life here."

Australia's Maritime Safety Authority (AMSA) said six merchant ships and three helicopters were at the scene, and 50 people had been rescued.

Papua New Guinea's maritime authority said earlier at least 28 people had been rescued, with many more floating in the sea near where the ferry went down.

The ship, MV Rabaul Queen, reportedly sank about 9 nautical miles (16 km) off Finschhafen on PNG's north coast while on its way from Kimbe on the island of New Britain to the mainland city of Lae, AMSA said.

An Australian search and rescue plane from the northeastern Australian city of Cairns, with multiple life rafts it can drop into the sea, also arrived at the scene.

The operator of the ferry, Star Ships, said it had lost contact with the MV Rabaul Queen earlier on Thursday, according to the Australian Broadcasting Corporation (ABC).

Papua New Guinea, Australia's nearest neighbour, is largely undeveloped, with poor infrastructure and limited facilities despite enormous resources wealth.

The majority of Papua New Guinea's six million people eke out subsistence lifestyles in villages clinging to jungle-clad mountainsides or scattered around its many islands.