Indonesia is examining whether a ferry that sank at the weekend may have had up to 103 more people on board than at first thought after many of their names did not appear on the manifest, an official said.

Desperate relatives chartered boats on Wednesday to join the search as bad weather continued to hamper rescue efforts after the 10-year-old Teratai Prima keeled over and sank off Sulawesi island in heavy seas.

The ferry had 250 passengers and 17 crew according to the manifest, but Bambang Ervan, a transport ministry spokesman, said at least 103 more could have been on board.

The official said it was too early to conclude that they were all additional passengers and some may have travelled under different names after somebody else bought their tickets.

"So, we are still verifying these matters," he said.

Only 36 survivors and two corpses have been found since the ship went down in the early hours on Sunday.

Some survivors were found after spending hours clinging to flotsam including, in at least one case, clusters of bananas.

Indonesian disaster investigators are focusing on the weather as the cause of a weekend ferry sinking, an official said.

Transport Minister Jusman Syafi'i Djamal said on Monday that a preliminary investigation showed the ferry capsized after it was hit by waves, but there would be an investigation into why the captain set sail despite warnings about bad weather.

Members of the National Transport Safety Committee had questioned the ferry's captain, who was one of the survivors, as well as crew on other ships in the same area at the time, an official said.

"What we are focused most on now is the weather, because the survivors were not able to grab anything, meaning that it happened really quickly," Kunto Prayogo, the chief investigator, told Reuters.

The ferry was travelling from Pare-Pare on the west coast of Sulawesi to Samarinda city on Indonesia's side of Borneo island.

With no more survivors found since Tuesday, anger and frustration has grown amongst relatives of the missing,

In Majene, a town north of the port of Pare-Pare, some relatives pooled money to rent boats to join the search.

"We are not satisfied with the search and rescue. We want to look ourselves," Azwar, who said he had family members on the ferry, told Detik.com news portal.

Waves of 4 - 6 metres (13 ft - 20 ft) hampered rescue efforts on Wednesday, although a slight improvement in conditions allowed more rescue ships and planes to be deployed, officials said.

Many people may have been unable to escape because they would have been asleep in cabins at the time, officials have said.

One survivor, Syaiful, described panic on the ship as it went down, with children screaming as people flung themselves into the sea, the Suara Pembaruan reported.

Authorities have denied the ferry was overloaded, although a transport ministry official said this week that the captain was not able to sound a general alarm to alert passengers.

The ferry had 12 inflatable life rafts, although there may have been no time to launch them, according to an official.