Heat oil in pan. Add garlic and prawns and stir-fry for 2 minutes. Add onion, zucchini and capsicum, stirring for 2 more minutes. Remove from stove.
In another pan, place curry paste and stir constantly for 1 minute. Add coconut milk and bring to the boil. Mix cornflour with water and add sugar, fish sauce and prawns with vegetables. Garnish with coriander leaves and serve with steamed rice.