Place the sugar and water in a small saucepan over medium heat. Stir until sugar dissolves. Increase heat to high, bring to the boil and cook for 2 minutes. Set aside to cool.
Carefully scoop out the rockmelon flesh and place in a food processor. Process until smooth. Add the buttermilk, lemon juice and sugar mixture and process until well combined. Place the rockmelon halves in the freezer.
Pour the rockmelon mixture into an ice-cream maker and follow the manufacturer’s instructions until the sherbet is just firm. Spoon the sherbet into the rockmelon halves and freeze for 4 hours or until frozen. Serves 4.