A decision to bring a boatload of asylum seekers to the Australian mainland shows the government is worried about a High Court ruling, the Greens say.

Prime Minister Tony Abbott insists the handling of 157 asylum seekers detained for weeks at sea is not illegal, as the government is accused of folding before a High Court ruling.

The asylum seekers are due to reach land late on Saturday before being transferred to Curtin detention centre in Western Australia, but Mr Abbott said he could not confirm the movements.

However, the prime minister did offer some guarantees.

“What we do is always in accordance with our international obligation, it’s always in accordance with our legal obligations, it’s always in accordance with safety at sea,” he told reporters in Canberra on Saturday.

Mr Abbott said that while the government stuck to its commitment to stop the boats, its methods were as humane as possible.

The asylum seekers have been detained on board a customs vessel at sea since their boat was intercepted 27km from Christmas Island on July 7.

The decision to bring them to land pre-empts a High Court challenge against the detention.

“The government has folded on bringing the asylum seekers off the prison ship onto the mainland because they are afraid of what the High Court will say,” Australian Greens senator Sarah Hanson-Young said.

The asylum seeker group includes several children, and Senator Hanson-Young said they should not be taken to Curtin, which she described as a “hellhole”.

She also asked the government for an assurance that the asylum seekers would be given full access to legal support.

Immigration Minister Scott Morrison has confirmed the group will undergo identity checks at Curtin by Indian consular officials.

A deal with Australia has resulted in India agreeing to take back its citizens and will consider taking Sri Lankan nationals who are Indian residents.

He expected many of many of those detained would return home.

But Asylum Seeker Resource Centre chief executive Kon Karapanagiotidis says the agreement with India has raised serious concerns.

“It’s deeply problematic and I don’t think it’s fair. I question whether it is legal, and we have a legal process to deal with those questions,” he told the ABC.

Labor’s Jim Chalmers said Mr Morrison was focused on a narrow and grubby political strategy.

“These people should have been taken to Christmas Island much earlier,” he told Sky News.

But Mr Abbott said he was determined to make an example of this boatload of asylum seekers.

“This is the first boat which has got as close to success, if you like, as one has in many a long month.

“We are determined to respond to this one in ways which underline our absolute implacable opposition to people smuggling.”