Asylum seekers held on an Australian customs vessel at sea for weeks have been offered lifeboats and told to make their own way back to India, according to a lawyer for the group.

The boatload of 157, who Hugh de Kretser on Monday 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," de Kretser, who is executive director of the Human Rights Law Centre, told AFP news agency.

"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."

The group, which includes 50 children, were picked up by Australian authorities towards the end of June.

They spent weeks on a customs boat, mostly locked in windowless rooms, before they were taken to Australia around July 25, their lawyers say.

Scott Morrison, immigration minister, who says the Tamils 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.

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.

No navigation experience

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.

De Kretser claimed access to the group had been extremely limited, but said some told him they had been living in India for less than six months when they made the voyage.

He said they had spoken of a precarious existence in India, where they were unable to work and their children could not attend school, while some had fears about their safety.

"These 157 men, women and children have been subjected to a level of cruelty that has no place in modern Australia," he said.

Under Australia's policy for asylum seekers, designed to prevent deaths at sea, those arriving by boat are sent to Papua New Guinea and Nauru and denied resettlement in Australia even if found to be genuine refugees.

Australia also has a policy of turning boats back when it is safe to do so, while asylum seekers coming ashore in Indonesia have claimed Australian authorities put them on lifeboats and turned them back.