To butterfly the chicken, insert a heavy, sharp knife into the cavity and cut down the backbone. Open the chicken up, skin-side down, and flatten the breastbone. Cut the backbone from the chicken. Remove the ribs from the flesh.
Blend the marinade ingredients in a small blender and pour over the chicken. Cover and marinate for at least 2 hours in the fridge, turning over once.
Remove from the marinade and pat dry. Barbecue on a hot plate on medium heat, skin-side up, for about 15 minutes. Turn the chicken over, cover, and cook for about 25 minutes or until cooked through, or the internal temperature is 74°C. Remove to a platter. Cover and stand for 10 minutes before carving. Great garnished with extra rosemary.