The federal government will defend its right to keep refugees in indefinite detention by claiming a High Court judgment in relation to one Sri Lankan asylum seeker does not apply to boat people.

Fifty-five asylum seekers are being held in detention indefinitely because they have been found to be genuine refugees, so they cannot be returned to their home countries, but ASIO has given them an adverse security assessment, so they cannot be settled in Australia. No third country is willing to take them.

The High Court ruled in October that a 36-year-old Tamil man known as M47 had been refused a protection visa on the basis of an invalid regulation, in a decision that was widely expected to extend to the rest of the group.

But the Australian Government Solicitor has written to the lawyers representing another member of the group, S138, saying that the ruling did not apply to him because he was an ”offshore entry person”. He arrived at Christmas Island by boat in July 2009.

M47 had attempted to come to Australia by boat but was caught up in the 2009 Oceanic Viking stand-off, in which the passengers refused to disembark the Australian customs ship that rescued them when their own vessel started to sink.

He was ultimately flown into Australia and arrived on a visa that lasted less than an hour. S138’s solicitor, Kate Mills, said this meant that he was processed under a different section of the Migration Act.

The legal team defending S138 will argue that the High Court’s determination in relation to M47 applies to S138, but failing that they will challenge the legality of keeping people in detention indefinitely. This would require the court to revisit its landmark ruling of 2004, in which four of the seven judges found the government was entitled to hold asylum seekers in detention indefinitely.

The court avoided doing that in the case of M47, by instead finding fault in the way the government had applied its regulations.

Asylum seekers who have been given negative assessments by ASIO are not entitled to see the material on which the determination was made. But the government has appointed the former Federal Court judge Margaret Stone to review ASIO’s assessments of each asylum seeker who is being held in indefinite detention.