The Geocentric Datum of Australia (GDA) is the new Australian coordinate system, replacing the Australian Geodetic Datum (AGD). GDA is part of a global coordinate reference frame and is directly compatible with the Global Positioning System (GPS).
Due to differences in the way the two datums were created, the GDA94 coordinates of a point appear to be about 200 meters northeast of the AGD coordinates of the same point. The precise size and orientation of the difference varies from place to place. For more technical information on the GDA, go to http://www.icsm.gov.au/icsm/gda/