Lurking and keeping an eye on you Aussies does pay off sometimes.
As an ex fruiterer of some 30 ish years I can tell you that your Plums are best left on the tree untill ripe to eat.They should be almost blubbery and the stone should come out clean of the fruit. Basically, The longer they are on there the better they will taste.
They are sold hard in the supermarkets for one reason only. Shelf life. Its more often the case that they have been picked at such an immature stage that they never ripen properly.
If the juice dont run down your chin its not ripe.