Heat 1 tablespoon of oil in a large frying pan over medium-high heat. Add the red and green capsicum, and onion. Cook, stirring frequently until capsicum are hot, but still slightly crisp, about 5 minutes. Remove from the pan and set aside.
Pour the remaining 1/2 tablespoon of oil into the pan, and add the prawns. Cook, stirring occasionally, until pink and opaque. Reduce heat to low, and return the capsicum and onion to the pan. Stir in taco or fajita seasoning and water so that everything is evenly coated. Simmer until capsicum are heated. Serve in warm tortillas.