A High Court ruling that a group of 157 Sri Lankan asylum seekers were legally detained at sea by Australian authorities last year has been welcomed by Immigration Minister Peter Dutton.

The asylum seekers were picked up and held on a Customs boat for nearly a month last June after leaving a port at Pondicherry in southern India.

Around 16 nautical miles from Christmas Island, after their boat was damaged by fire, they called for help and were picked up by the Customs ship Ocean Protector.

They were detained on the ship and, under the Maritime Powers Act, after a direction from the Australian Government, the ship began sailing back to India.

But they were eventually taken to Nauru after efforts to return them to India failed.

Lawyers for the group argued the detention outside Australia was illegal and at odds with international obligations, and their treatment on the Customs boat was inhumane and cruel.

The plaintiff in the case was a Tamil asylum seeker known as "CPCF". He did not have a valid visa entitling him to enter Australia.

But he took the Government to court and claimed damages for wrongful imprisonment.

Asylum seeker advocates were concerned over changes to legislation made late last year that could make it difficult to challenge boat turn-backs and detention at sea in the future.

The changes meant detention powers could not be ruled invalid on the basis of international obligations.

Today the High Court found Australian authorities were not obliged to offer procedural fairness by asking the group if they were owed protection under international obligations.

The complex legal decision split the court bench four to three.

It also found the Federal Government acted lawfully and the group was not entitled to damages.

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Mr Dutton welcomed the High Court's decision and said the judgement proved the Government was acting within the law.

"It has vindicated the Government's position and we welcome the result," he said.

"We meet our international obligations and we at the same time have a difficult situation to deal with to stop people drowning at sea."

Mr Dutton said he would provide a more detailed response to the judgement's legal implications once the Government had read it thoroughly.

The asylum seekers were ordered to pay costs and will be processed on Nauru.

Human Rights Law Centre executive director Hugh de Kretser said that while the outcome was disappointing, the case had succeeded in bringing vital legal scrutiny to the Government's actions at sea.

"It took this case for the Government to finally break its secrecy and confirm that it was detaining 157 people ... on a boat somewhere on the high seas," he said.

"If it hadn't been for this case, the Australian public may never have known what happened to those 157 people."

Lawyers who brought the case to the High Court said at least one of the asylum seekers had already been found to be a genuine refugee.

But the Government remained firm that the asylum seekers would not be allowed to settle in Australia.

Last July, then immigration minister Scott Morrison said the asylum seekers were economic migrants and they would not face persecution if they were sent back to India.

The Government has now changed the law to make any future legal challenges more difficult.