The boatload of 157, who lawyer Hugh de Kretser said were mostly Christian Tamils fleeing persecution in Sri Lanka, set sail from India hoping to get to Australia.
"The clients we spoke to were absolutely terrified at what lay ahead for them," said de Kretser, who is executive director of the Human Rights Law Centre.
"They were terrified of the prospect of being dumped in the ocean on lifeboats, without experience in navigating or operating a boat and having to take responsibility for the families that were on the boat."
They spent weeks on a customs boat, mostly locked in windowless rooms, before they were taken to Australia around July 25, their lawyers say.
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Immigration Minister Scott Morrison, who claims they are mostly economic migrants, said they could be returned to India -- even if not citizens of that country -- under an agreement with New Delhi.
But all refused interviews with Indian consular officials on Australian soil and were instead transferred to a detention camp on the Pacific island of Nauru.
Morrison has previously refused to detail "on water" operations of Operation Sovereign Borders, the government's policy designed to break people-smuggling and stop asylum boats.
De Kretser said that after interviews with 15 of the 107 adults onboard, it appeared nine of them were separated from the others while they were still on the customs vessel and told how to use the lifeboats.
They were instructed in English that there would be 50 to 60 people on each boat and they would have to navigate them back to India.
"When they refused, saying they had no experience in operating or navigating a boat, and couldn't take responsibility for ensuring the safety of those onboard, the officers told them that it was an Australian government decision and they had to obey," he said.