Leaf spot disease of escallonia has appeared in many parts of the country in recent years, causing significant or severe leaf drop. Spraying for this disease is not really practical and all you can do is leave it be and see how it progresses. This disease is generally worst in wet weather, high rainfall areas, and on the plant close to the ground where the leaves stay damp longer.
Sometimes diseases have a very favourable spell of weather and afterwards, they do not present the same problem again, but often the problem becomes chronic.
It appears to be quite virulent and causes severe defoliation and weakening, leading to death, of plants.
It would appear to be a fungus called Mycosphaerella or related Septoria leafspot.
Escallonia leaf spot is here to stay and will continue to be a problem. Try applying a fertiliser with high potash that might harden the leaves but it is unlikely to do much and spraying is not a solution, as it is not possible to spray large bushes, or to spray often enough for control.