(CNN) -- Rescuers were searching Friday for nearly 120 people unaccounted for after a passenger ferry sank off the east coast of Papua New Guinea, authorities said.
The 238 known survivors arrived by five vessels at the city of Lae. Many were undergoing medical tests, according to Capt. Nurur Rahman, rescue coordinator with the country's National Maritime Safety Authority.
About 350 people were aboard the MV Rabaul Queen when it sank Thursday, said Rahman.
The MV Rabaul Queen went down about 16 kilometers (10 miles) off Cape Fortification in the Vitiaz Strait after getting into trouble early Thursday, officials said.
The ferry is about 1,000 meters (3,280 feet) below the surface, Rahman said.
After an alert was sent out, boats and helicopters rushed to the scene to try to save scores of people left adrift at sea by the sinking. They had to deal with high winds and ocean swells.
Joanne Meehan of the Australian Maritime Safety Authority told CNN two of that country's aircraft were assisting in the search.
Australia is Papua New Guinea's closest neighbor minus Indonesia, with which it shares a border.
"I'm pleased that a large number of people have been rescued," said Kevin Rudd, the Australian foreign minister. "We're deeply concerned about those who are still missing."
Rudd said he didn't have any information on the nationalities of the passengers aboard the ship.
"This is a difficult operating environment," he said. "We are doing everything within our power."
The Rabaul Queen was ferrying passengers from the town of Kimbe on New Britain Island to Lae when it got into difficulty.
"Taking ferries is the most economic method of transport for people traveling between the islands and the mainland," Rahman told CNN. "What I understand from local media is that a lot of students and their parents were traveling to schools that recently reopened on the mainland."
Jurgen Ruh, the chief executive of Manolos Aviation Ltd. in Lae, was in one of the helicopters that helped with the rescue effort.
"I could see the survivors on the debris," he said, noting that the passenger ship had sunk completely by the time he reached the scene.
There were about 10 life rafts visible, each with roughly 10 people on them, the first time his helicopter flew overhead, he said. However, when the helicopter returned to the area after refueling, no more survivors were spotted.
The weather was likely to be a contributing factor in the ferry's sinking, Ruh said, as there were high winds at the time in what is a notorious area.
Papua New Guinea had a population of 6.7 million in 2010, the U.S. State Department says, with about 190,000 people living in Lae. Most of the population is scattered in small settlements across the state's many islands.
The largely Christian country is part of the Commonwealth and has Queen Elizabeth II as head of state. The bulk of the estimated 2,000 American residents in Papua New Guinea, are missionaries and their families, the State Department says.
CNN's Elizabeth Joseph, Karen Smith, Jethro Mullen, Phil Gast, Laura Smith-Spark and Judy Kwon contributed to this report.