The Christmas Island Detention Centre has cost taxpayers $27million since being reopened this year, despite holding just four detainees.
The Morrison government recommissioned the facility in February, over fears of an influx of asylum seekers tied to changes to refugee medical transfer laws.
Home Affairs Department officials told an estimates committee on Monday that over 100 staff have been on hand at the detention centre, which has housed the Biloela Tamil family at the centre of a legal fight over their deportation since late August.