A High Court challenge to the Abbott government's powers over asylum seekers it takes into custody on the high seas has been widened to include a claim for damages for false imprisonment.

Lawyers for more than 150 asylum seekers who are now being held at the Curtin detention centre in remote north Western Australia have urged the court to continue hearing the case, insisting the Abbott government “can't engage in conduct like that with impunity”.

“They were detained for almost a month on the high seas pursuant to a power we say the government did not have,” Ron Merkel QC told Justice Kenneth Hayne on Monday.

Although there was no longer an urgent need for a hearing by the full court next week, Mr Merkel argued that the issues in question were still live and in need of clarification because they went to Australia's obligations under international law.

“I remind your honour that the government's policy of making no comment on what is referred to as 'operational matters', means that the events that occurred in this case could occur without anyone knowing, without anyone having the opportunity for judicial intervention,” he said.

The bottom line was whether the government had the power to return people to places they were seeking protection against without having their claims for protection assessed, he said.

While counsel for the government argued further proceedings be remitted to the Federal Court, Justice Hayne said he considered it premature to make such a determination.

The judge gave the government until 4pm on Thursday to reply to an amended claim by lawyers for the asylum seekers, giving both parties the ability to then apply for a directions hearing.

“I'm not going to just let this case just lie on the file. Counsel should order their affairs on the assumption that once the pleadings are in I, or another justice of the court, will be looking at it with some care.”

Immigration Minister Scott Morrison, who says the asylum seekers were taken to the mainland so that Indian consular officials could establish how many were Indian citizens, has been accused of pre-judging the claims for refugee status.

“We are talking about people who are seeking to come to Australia from a safe country of India and the suggestion that they are anything other than economic migrants here I think defies belief and credibility and the Australian government is just not going to put up with that,” Mr Morrison said.

Mr Hugh de Kretser, of the Human Rights Law Centre, said Mr Morrison had admitted in court documents that the asylum seekers were claiming refugee protection.

Lawyers representing the asylum seekers, who overwhelmingly are Sri Lankan Tamils who departed from India on June 13, say it has been impossible to assist in putting their claims.

"We've been given very little to no access to our clients. And I am extremely concerned that they are about to be interrogated by foreign powers and possibly delivered up to them without [us] even having the opportunity to speak to them," said George Newhouse.

"I can't tell you anything about these people and their claims because I haven't had the chance to speak with them. No-one has. The government is still holding them as virtual prisoners."

Mr Morrison has not ruled out sending the asylum seekers who are not returned to India to Nauru for detention, saying: "We reserve all of our rights for offshore processing because it doesn't matter whether you come to Christmas Island or you come to Curtin, you can still transfer people to Nauru from Curtin."

A spokesman for the Nauruan government said it was "always ready to accept asylum seekers, however the movements of asylum seekers are at the discretion of the Australian government and all questions should be directed to them".

In 2012 the Gillard government passed legislation that made "illegal maritime arrivals" who reached, or were taken to the Australian mainland, eligible for offshore processing.

With Sarah Whyte and Ben Doherty

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