This app produces a map showing you the catchment boundary of a selected stream or river reach in the Melbourne region, and offers you the option of downloading the boundary polygon as an ESRI shapefile or as a MapInfo file.
It uses the 'subc' field in Melbourne Water's DCI subcatchment layer (2017 version) to calculate the catchment of any subc in that table that has a stream flowing through it. The full stream layer (with which you can find the subc code for any reach in the region) is available for download as an ESRI shapefile here

You can either open that file in a GIS program and select your reach(es) of interest there, or enter the subc of one of the terminal reaches listed below, that will show you the catchment and its stream network. You will be able to click on any reach displayed as a dark-blue line, to find its subc. You can then enter that subc to calculate its catchment. (Press delete to enter a different subc)...or if you're feeling lucky, try a subc code at random.

The table below lists subcs for reaches at the bottom of catchments across the region. These are the largest catchments for which plotting stream lines doesn't take too long. If you select one of these you will be able to click on any reach upstream (or downstream) of it to find the subc of the reach you are interested in. You can also select larger catchments, but the app won't plot the stream lines. If you are interested in a stream not covered by this list, you'll need to use the stream layer in a GIS program to find the subc code.