The Government has confirmed it intends to send 157 Sri Lankan asylum seekers who have held been on a Customs boat to Curtin Detention Centre in remote WA.

The group of men, women and children left in a boat from India and have spent nearly a month in legal limbo on board the Customs ship after being intercepted by Australian authorities.

Previously the Government has only said it is taking the group to the mainland so Indian officials can assess whether they want to take them back.

The asylum seekers will be taken to Cocos Island and then flown to the Curtin centre.

The office of Immigration Minister Scott Morrison says it cannot reveal exactly when the group will arrive.

The ABC understands a charter flight was arranged to collect the group from Cocos Islands but the charter company's flights are not monitored by flight tracking.

Mr Morrison has insisted no asylum seekers who come by boat would reach the Australian mainland, but he has been forced to make an exception because India wants to interview them.

Mr Morrison says the safest and most convenient way of doing that is in Australia, although he insists they will never be settled here.

He says after Indian officials speak to the asylum seekers, the country will accept back those who are Indian citizens and possibly also Sri Lankan citizens who have been living in India.

Opposition foreign affairs spokeswoman Tanya Plibersek says they should have been brought to Australia weeks ago.

"As the vessel was close to Christmas Island, it made sense to process people on Christmas Island," she said.

"It's only been Scott Morrison's ego that's prevented that."

'Would we allow ISIS to interrogate those who left Iraq?'

Greens Senator Sarah Hanson-Young says there is no legal basis for allowing the Indian officials to question the asylum seekers.

"These people are in Australia's care clearly. They're on Australian soil," she said.

"What's next? Will we be allowing the Sri Lankan government to come in and interview the Tamils? Would we go so far as to allow ISIS to interrogate those who left Iraq?

"This is about a proper process of law and doing what is right by those who are most vulnerable.

"These children, their families, have been at sea for weeks. They've been used as political footballs by this Government and now we have Tony Abbott wanting to simply hand pass them over to another country."

High Court to hear asylum seekers' case

The High Court is likely to hold a directions hearing next week to examine the implications of the Government deciding to bring the Tamils to the mainland.

The case has been held while the asylum seekers were on the Australian boat at sea, but Hugh de Kretser from the Human Rights Law Centre, who represents the asylum seekers, says the principles of the case are still relevant.

"The case in the High Court really boils down to two simple propositions: one is that the Australian Government ... could not intercept this boat and return them to a place where they wouldn't be safe ... secondly, that there needed to be a fair decision-making process around that," he said.

"They're the two key concerns of these clients and they will still be relevant in some context in relation to the move to bring them to mainland Australia."

He says it is too early to comment on whether a return to India would prompt further legal action.

"We need to see what the Government is proposing and then provide advice to our clients about what's actually proposed," he said.

Meanwhile Prime Minister Tony Abbott has reiterated his message that those coming by boats will never get permanent residence.

Mr Abbott says the Government will do whatever it legally can to stop boats coming to Australia.

"This is the first boat which has got close to success if you like. We are determined to respond to this one in ways which underline our our absolute implacable opposition to people smuggling," he said.