Preheat oven to 450 degrees F (230 degrees C). Spray a large baking dish with cooking spray.
Pour milk into a bowl. Place flour on a plate. Pour panko into a shallow bowl.
Dip barramundi in milk, dredge in flour, and coat with panko. Place in the prepared baking dish. Spray barramundi with cooking spray. Sprinkle garlic powder and parsley on top.
Roast in the preheated oven until barramundi flakes easily with a fork, about 12 minutes.
