The Nauruan group, which was acquitted in September of charges over an anti government protest in 2015, is heading back to court.
They are to face an appeal lodged by the Nauru government.
Called the Nauru 19 the group had been through a convoluted legal process for more than three years.
In September, Justice Geoff Muecke, the retired judge brought in by the Nauru government specifically to hear the case, ruled there should be a permanent stay on the people facing trial.
This followed an application for a stay because of time the legal process had dragged on and the refusal of the government to meet some of their legal expenses.
The director of public prosecutions had indicated straight after Justice Muecke's decision that there would be appeal and now this has been formally lodged.
It will be one of the first matters to come before the new Nauru Court of Appeal, set up after Nauru ended its access to the Australian High Court as the final appellate court earlier this year.
The chief justices from Vanuatu, Solomon Islands and Kiribati have been appointed to the new court, along with a senior judge from Papua New Guinea.