Amnesty International is calling for the 153 people, now entering their third week detained in windowless rooms on board a boat, to immediately be brought to Australia for processing.
The Sri Lankan asylum seekers were intercepted by the Australian Navy on their way from India.
"We have grave concerns at reports from lawyers that they must request permission to move from one room to the other and can only leave their rooms in the presence of a guard," said Graeme McGregor, Amnesty International Australia, Refugee Campaign Coordinator.
Amnesty International understands the asylum seekers have also had personal items seized, including mobile phones and families have been split, with men and women separated.
"Detaining men, women and children on a boat, locked in rooms without windows and severely restricting their communication with the outside world with no access to translators, is inhumane.
"Throughout this process, the Government has continually failed to provide basic answers to the questions about the asylum seekers' whereabouts and their safety.
"It took a High Court challenge for the Government to even confirm the boat exists.
"The Australian Government must answer the question: how far is too far when it comes to 'stopping the boats'?"
Amnesty International continues to condemn the secrecy surrounding offshore processing, calling for a more humane asylum seeker policy and an end to offshore processing.
The human rights organisation has also issued a global Urgent Action to 4.6 million supporters calling on the Australian Government not to return these asylum seekers to Sri Lanka, where they could face serious human rights violations.