Peel and devein prawns; reserve the heads and shells. Refrigerate prawn meat in a large bowl until required. Peel and finely chop onion and garlic; reserve skins. Trim celery; finely chop, reserving the trimmings. Remove rosemary leaves from the stalks; finely chop leaves, reserve the stalks.
Heat 1 tablespoon of the oil in a large saucepan over high heat. Add prawn heads and shells; cook, stirring, for 2 minutes. Add reserved skins and trimmings from the onion, garlic and celery, with reserved rosemary stalks. Add the water and ½ cup (125ml) of the wine; bring to the boil. Skim any scum from the surface. Reduce heat; simmer, covered, for 30 minutes. Strain stock into a heatproof bowl; discard solids. Skim any additional scum from the surface. Keep warm.
Heat 2 tablespoons of the remaining oil in a large saucepan over medium heat; cook chopped onion, garlic and celery, stirring, for 5 minutes or until softened. Add rice; cook, stirring, for 1 minute. Add remaining wine; cook, stirring, for 1 minute. Reduce heat to low. Add stock, one ladle at a time; cook, stirring, for 25 minutes or until stock is absorbed and rice is just cooked.
Meanwhile, brush corn with a little of the remaining oil; cook on a heated grill plate (or grill or barbecue) over high heat for 3 minutes each side or until cooked. Remove from heat. When corn is cool enough to handle, cut kernels from cobs. Add remaining oil to prawn meat, season; toss to combine. Cook prawns on heated grill plate for 3 minutes or until cooked.
Add prawns and corn kernels to risotto; cover, cook for 5 minutes or until prawns are cooked through. Stir in butter and chopped rosemary; season to taste. Serve risotto topped with parsley.