Drought? Magnesium shortage? Potassium shortage? Need more info I'm afraid - do the leaves go completely yellow, or just between the veins? Or just at the edges? Do they then go brown and crispy, or fall off when they're soft but yellow? Does this happen shortly after the plant starts into growth in spring, or later? What kind of hydrangea are you talking about? In pots or in the ground, planted near trees, in full sun or shade? How long have you had the plant/s?

at this time of year I'd be tempted to assume natural leaf fall .

Here, Seaburngirl, but not in Australia.