For brine, combine ingredients and 4 litres of water in a large stockpot, bring to the boil and simmer for 10 minutes. Strain and cool.
Submerge turkey in brine and weight down if necessary. Refrigerate for at least 24 hours. Five hours before cooking remove turkey from brine and rinse under cold running water. Pat dry with absorbent paper.
Preheat oven to 180C. Place turkey breast into roasting pan and rub all over with butter and season with sea salt and freshly ground black pepper. Roast, basting frequently, for 1 hour and 20 minutes or until golden brown and just cooked through. Temperature should read 65C on a thermometer inserted into the middle of the breast.
Cover turkey loosely to keep warm and rest for 10 minutes, then carve and serve drizzled with pan juices.