Melbourne: More than 200 people have been rescued - most of them from the waters off Papua New Guinea after a ferry sank on Thursday but many others remained missing, a top rescue official said.

"About 200 people had been rescued - most of them from the water," Rescue co-ordinator Captain Nurur Rahman said, adding that several passengers were still unaccounted for.

Captain Rahman said four merchant ships were diverted to the scene by Australian authorities to help with the rescue. Two helicopters from Lae had also joined the rescue operation.

The rescuers are scouring the area for further survivors, he said, adding the priority at the moment is to save lives.

He said it was too early to say why the ferry went down.

The Australian Maritime Safety Authority (AMSA) said the ferry MV Rabaul Queen, with about 350 people on board, went down 16 kilometres off the coast of Finschhafen, which is about 80 kilometres from its final destination of Lae.

Ferry operator Rabaul Shipping was quoted by the ABC news as saying it lost contact with the vessel early this morning.

AMSA put a call out for ships in the area to head to Finschhafen. "We've sent three planes at this stage - one of them is already on scene, and that's one of AMSA's contracted dedicated search and rescue Dornier aircraft from Cairns," AMSA spokeswoman Carly Lusk said.

Police in Kimbe, where the ferry sailed from, said most of the passengers were students and trainee teachers, an agency reported.

Earlier, Australian Prime Minister Julia Gillard said that the death toll could be very high.

"There has been a boat go down in Papua New Guinea with, as we understand it, 350 people on board," Gillard said.

"This is obviously a major tragedy... There is likely to be a very high death toll," she said.