To prepare the brine, place the salt, sugar, garlic, fennel, juniper, peppercorns, half the lemon slices, tarragon, bay leaves and I litre of water in a medium saucepan over high heat and bring to the boil. Cook for 10 minutes, stirring to ensure the salt and sugar is dissolved.
Pour brining liquid into a large (approx. 10-litre-capacity). Add the remaining 7 litres of water and stir to combine.
Using your hands, carefully loosen the skin of the turkey at the breasts. Tie the legs with kitchen string and tuck wings under.
Place the turkey breast-side down in the brining liquid. Cover and refrigerate for 6 hours.
Remove the turkey from the container, discarding the brining liquid, and place on a lightly greased wire rack over a deep oven dish.
Place the butter, lemon rind and crushed garlic in a small bowl and combine.
Carefully spread the butter under the loosened skin of the turkey. Place the remaining lemon and extra tarragon inside the cavity.
Preheat oven to 180˚C. Pat the turkey dry with paper towel and brush with oil.
Pour extra water into the dish and cover with lightly greased aluminium foil and roast for an hour.
Uncover and roast for a further 45 minutes or until the skin is golden and the juices run clear when tested with a skewer.
Cover with foil and allow to rest for 20 minutes before serving.