The Federal Government may be forced to reveal some details of its secretive operation to hand back asylum seekers to Sri Lanka in a High Court hearing underway in Melbourne.

Lawyers acting for about one third of the 153 asylum seekers, whose whereabouts the Government refuses to disclose, are seeking to challenge the legality of the Commonwealth's actions.

The High Court is yet to decide if the lawyers have a case, but the hearing may for the first time make public some of the details of the contentious operation.

The Government has already sent back one boatload of asylum seekers to Sri Lanka and is in the process of trying to do the same with a second, much larger group.

Lawyers representing 48 men, women and children are looking to mount a High Court challenge. The asylum seekers are part of a boatload of 153 Tamils who were last in contact with the outside world more than a week ago.

First, the lawyers need to convince the High Court there is merit in the case. Until that is decided, the court has issued an interim injunction to block the Government from handing any more asylum seekers over to Sri Lanka.

Asylum seekers held without due process says lawyer

Sydney solicitor George Newhouse is one of the legal team arguing their clients' asylum claims are not being properly assessed and they are being held in custody without due process.

"The basis of our case is that the Australian Government is not allowed to hold men, women and children in a secret rendition location under our law," he told The World Today.

"They have to deal with people in accordance with the law.

"We say they have not been processed properly, they did not come into the Australian migration zone, they aren't being processed in the normal way and they aren't being kept in the normal way. And we are entitled, on their behalf, to question on what basis, on what legal basis, they're being held."

Mr Newhouse said the asylum seekers were "quite possibly" being held unlawfully.

"It's not clear at this stage whether it's lawful or not because nobody knows where they are or on what basis they're being kept. That will come out in these proceedings."

Asked if he was sure the High Court had jurisdiction over the case, even though the asylum seekers may be in international waters, he said: "Well the minister [Scott Morrison] certainly seems to think that he's got the jurisdiction. If he's got the jurisdiction, and they're in the hands of the Australian Navy, then certainly the High Court would have jurisdiction over them as well."

The Attorney-General's office would not say if the Commonwealth would attend this afternoon's hearing. The Australian Government solicitor's office says it will attend.

If nothing else, the legal action might flush out who is holding the asylum seekers, and the circumstances under which they are being held; under what powers; whether they are in custody or in detention; and whether that is lawful.

The Government maintains its secrecy is to stop people smugglers plying their trade. That may not be enough to satisfy the High Court.

Backbencher unable to say if he backs policy

The lack of detail also makes it difficult for some Coalition MPs to comment on the Government's approach to assessing asylum seekers' claims at sea.

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LNP backbencher Bert van Manen was unable to say whether he backed his party's move or not.

"I don't have the full details of the processes that have been used," he said. "That is a question you'll have to ask the minister, Scott Morrison."

While the Government insists it is adhering to all its international obligations, human rights agencies, lawyers and advocates argue otherwise.

The United Nations High Commissioner for Refugees says that without further information it is not in a position to confirm whether Australia is fulfilling its obligations, although is remains "deeply concerned".

Greens Senator Sarah Hanson-Young accuses the Government of holding the public and the parliament in contempt with its secrecy, but says that may not work in the case of the High Court.

"It will be a test of the legality of the Government's ability to hold people in custody out on the high seas," she said.

"It may be that they've already handed them back [in]to the hands of the people they're fleeing from. But we suspect that they are being held in custody on [a] Customs vessel. We suspect that that includes all 153, including the 37 children. And rather than playing hide-and-seek with people's lives, the Government is now going to have to front up to the court and explain what it's doing. That has to be a good thing."

Late yesterday a man who said he was related to a three-year-old girl on the boat issued a statement pleading with the Government to give details of what has happened to her.

The Tamil Refugee Council issued the statement from the man along with a picture of the girl, who it named as Febrina.

"I am desperate to know where my family is. I can't function at all not knowing. I know all of them would be in very big trouble if sent back to Sri Lanka," the man said in the statement.

"I want to plead with the Australian minister to stop our pain and let us know what he has done with all the kids and families on the boat. I ask him to be kind to these people. They are all very frightened. They cannot be sent back to Sri Lanka. Many of them will be tortured again and even killed."