The Nauru Supreme Court is hearing a case on the legality of detaining asylum seekers on Australia's behalf.
The hearing is to test the validity of the visas, which require asylum seekers to remain locked up for all their time in Nauru.
The hearing comes despite the Nauru justice minister, Russell Kun, ordering Australian senior counsel appointed to represent the Nauru and Australian governments to return home.
Earlier, the minister revoked the visas of the Australian lawyers to have acted for the asylum seekers.
A paralegal pleader, Reuben Kun, a former finance minister and the uncle of Mr Kun, was appointed to represent the detainees instead.