An agreement between Australia and Nauru to set up an offshore detention centre doesn't mention that asylum seekers must be detained, the High Court has been told.
Nor does a contract between the Australian government and the centre's service provider Transfield.
As a result the Australian government is not responsible for detaining asylum seekers on Nauru, its lawyers argued on Thursday.
That missing requirement forms part of the Commonwealth's defence in a case testing the legality of offshore processing centres.
The case is being prosecuted on behalf of a Bangladeshi asylum seeker who was brought to Australia from Nauru for medical treatment during her pregnancy last year.
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The plaintiff's case will test whether the federal government can return the woman - who is protected by the constitution while here - to an offshore centre run by Australia.
Her lawyers also argue no Australian laws allow her return.
The Commonwealth's case in part hinges on a four-day-old decision of the Nauru government to allow asylum seekers to roam around the island at will.
The detention facility will now have an a permanent open-door policy - a relaxation of the previous 12 hour, five-day-a-week arrangement.
Commonwealth Solicitor-General Justin Gleeson said that policy change meant the woman, if returned, would not be detained.
"Depravation of liberty does not equal detention in custody," he told the court on Thursday.
The Human Rights Law Centre, which has brought the case, says the timing of that decision is suspicious.
Mr Gleeson said even if detention could be proved, it could not be found to be controlled by the Australian government.
Australian law required the asylum seeker be removed, but did not prescribe detention once they arrived at an offshore destination.
But the woman's lawyers argue the centre and legal regimes exist "only" to accommodate and detain asylum seekers while they are being processed.
Her counsel Ron Merkel pointed to sections of the Transfield contract to argue the immigration department had control over the service provider.
He claimed a section that allowed searches to be conducted at the department's request and its step-in rights to suspend services proved the Commonwealth had control.
Mr Gleeson rebutted that as a "very large error", pointing out Transfield staff answered to a Nauru-appointed operations manager.
"Large and small, everything traces back to the operation manager," he said.
The plaintiff's contention that the operations manager "lived in a helicopter and didn't have control" was incorrect.
The federal government, with Labor's support, in June retrospectively changed laws to shore up the Commonwealth's ability to pay for the offshore facilities.
Mr Gleeson argued the parliament passed the law so that part of the case "didn't trouble the court".
The outcome could affect the future of 200 other asylum seekers who have been transferred to Australia for medical treatment or been victims of assault and violence.
