Are you in that extreme heat belt that some of the other Aussies are having? If so, are you watering more often? Blossom End Rot is not usually a real calcium deficiency, it's usually a temporary deficiency caused by environmental factors. One of those is heavy watering after a dry spell, so it is possible that, if the plants are getting very dry, you might want to try Hippy Seed Company's method of watering with a soda bottle stuck in the ground.
If you want to fertilize, use a fertilizer with a low 'N' number, and use one that is complete, that is, one that contains all the micro nutrients, too.