Rosemary bush is happy but grows uneven to one side

The most likely reason for a plant to grow unevenly is due to a lack of sun. If you notice the side of the plant receiving less light is more sparse, then this is the reason for the unevenness. For potted plants, turn the pot every now and then to expose all sides of the plant to sunlight. For plants in the garden, prune nearby plants to increase sunlight availability. Lightly tip prune to encourage bushiness and fertilise to promote growth. A great fertiliser for Rosemary is Yates Thrive Natural Vegie & Herb Organic Based Pelletised Plant Food.