Spatial data (with x and y coordinates) is set out in a number of possible map projections. When DRAINS imports a background and other data from CAD or GIS files, these will be to one of the common projection systems used. Generally, you will not need to specify a projection unless you are working with MapInfo imports and exports (ESRI and CAD programs do not ask for projections to be named).
The projections likely to be used in Australia are:
Map Grid of Australia 1994 (MGA94), which is the recommended grid for new work,
Australian Map Grids (AMG66 and AMG94), older systems that are obsolete, and
State map grids such as ISG in New South Wales, that are also obsolete.
You may use plain x-y coordinates, which are termed 'Non-Earth' in MapInfo. Further information can be found in GIS manuals and Help systems and on the Internet.