Hebes don’t like high phosphate fertiliser for they are shallow rooting plants. If you have fertilised lately this could be the reason they are dying. I would recommend using an organic fertiliser or recycled animal droppings in the future. It could also be phytophthora that is killing your hebes. Phytophthora is a disease that destroys the root of the plant and is spread by high quantities of moisture in the ground. This may have destroyed the roots in late spring when it was very wet and we had a high water table. There is no cure for this disease. However, you can prevent this by using a plant more suited to a high moisture condition or place tricopel in the hole as you plant.
Other plants in your garden may also show signs of phytophthora with the odd branch dying back. I suggest pruning these off.Also check that the hebes have not been attacked by spidermite. These can be quite common in summer if it was a long hot dry spell which theses insect thrive in. The evidence of these insects is generally a line web on the back of the leaves and a maroon colour to the top of the leaf. The best cure I can recommend is Yates Super Shield.