Juice half the watermelon in a juicer, or process in a food processor then strain, and refrigerate until required. Scoop melon balls from remaining watermelon with a melon baller, place on a tray and freeze overnight.
Combine rhubarb, sugar and 250ml orange juice in a saucepan, bring to the simmer over medium heat and cook until rhubarb is just tender but still holding its shape (8-10 minutes), then refrigerate until required.
Combine watermelon juice, frozen watermelon balls and poached rhubarb in a punch bowl with remaining orange juice, ginger wine, gin and Cointreau. Top up with tonic water to taste and serve.