The Abbott government has promised to give three days’ notice before trying to return to Sri Lanka a group of 153 asylum seekers, who are being held aboard a Customs vessel outside Australian territorial waters.

In the High Court on Tuesday, the government for the first time confirmed their existence, amid rumours in the past week of a boat carrying children.

The United Nations has also called for a judicial review of Australia’s refugee policies, providing an additional threat to the government’s flagship ‘stop the boats’ commitment.

Click here to find out why the United Nations thinks Australia needs a wide-ranging judicial review into its refugee policies, including a check to see if it is complying with the UN Convention against Torture.

The group, which includes children as young as two years old, was intercepted at sea en route to Australia but won’t be processed under the Migration Act because they never entered territorial waters – a decision slammed by Labor and the Australian Greens.

George Newhouse leaves the High Court in Sydney.

George Newhouse from Shine Lawyers, who is representing 48 Tamils on board the Customs vessel, said while government was “showing some compassion” by undertaking to give three days’ notice of any intention to return them, their future remained uncertain.

“It is only a temporary fix, but it’s blown a bit of a hole in the government’s treatment of asylum seekers.”

The group has been on board the Australian Customs vessel in the Indian Ocean since being intercepted in the last week.

Late on Monday, the High Court granted an urgent temporary injunction preventing the return of the 153 to Sri Lanka ahead of Tuesday’s hearing in Melbourne, where Commonwealth Solicitor-General Justin Gleeson undertook to give 72 hours’ notice before moving them.

Refugee rights activist Ian Rintoul said the decision would come as an “enormous relief” to the asylum seekers.

“It is only a temporary fix, but it’s blown a bit of a hole in the government’s treatment of asylum seekers,” he said.

The government had previously declined to reveal any details about the boat, citing its longstanding policy not to comment about on-water operations.

But Mr Gleeson told the High Court on Tuesday that Australian authorities had intercepted the boat 12 miles outside Australian territorial waters.

This means the asylum seekers’ future would be determined by the Maritime Powers Act, not the Migration Act, he said.

• Mental health fears as more children sent offshore

The matter will return to court within 21 days for a directions hearing.

This is the second recent asylum seeker incident, with another boat intercepted off the Cocos Islands in late June.

Sri Lankan asylum seeker children returned by Australia wait outside a court in Galle. Photo: Getty

The 41 asylum seekers on that boat were handed back to a Sri Lankan navy vessel after screening at sea by Australian authorities.

The government said the group of 41 did not risk persecution if returned.

In Galle, Sri Lanka on Tuesday, the group faced court on charges of attempting to leave the country illegally. Twenty-seven were granted bail, five were remanded in custody and nine children were discharged.

The United Nations High Commissioner for Refugees has criticised Australia’s treatment of these asylum seekers, saying the UN’s experience with onboard processing has generally not been positive.

“Such an environment would rarely afford an appropriate venue for a fair procedure.”

“Such an environment would rarely afford an appropriate venue for a fair procedure,” the UNHCR said in a statement.

Labor immigration spokesman Richard Marles also expressed grave concerns about processing at sea.

“The only reason this has not occurred is to protect this government’s political scoreboard,” he told reporters in Melbourne.

Greens Senator Sarah Hanson-Young said the government should allow asylum seekers to be processed in Australia.

“I don’t want children detained on a prison ship out on the high seas,” she told reporters. “Let’s get them off that ship first and talk about what appropriate and humane treatment they should be given once they have set foot on land.”

– with AAP