In my Caroma Concorde cistern the bottom flush valve had perished. Thanks for your helpful advice about how to remove the old washer (flat rubber disk with central hole) simply by stretching it and pulling it off the groove in which it fits.
But to get there you have to take out the mechanism around it. Easy once you get it. Mine had two slots at the back and two flat tabs at the front with a central built-in plastic stud that locks them in. Secret: Start at the back; grab each corner of the mechanism with pliers in turn and check that they can be eased out upwards. Once they are loose, separate the locked tabs gently, one at a time, by bending them in opposite directions until their central pins pop out.
I bought a pack of 4 washers from Fixaloo.com.au suitable for Caroma for $6 approx, and only needed 2. I decided to replace the inlet flush valve too. Note this needs 2 washers - a small central black one like a tablet and a red disk one that goes outside. You can pop out the old central black plug using a pointed object and replace it with the new one. Then fit the wider one around it.
Barry A, Melbourne, March 2011