Heat oil in a large frypan over medium heat. Saute onions until soft. Stir in garlic and saute about 1 minute. Add tomatoes and cook until they begin to soften. Stir in wine, olives, capers and chilli flakes. Heat to a simmer.
Place barramundi into sauce. Cover and gently simmer for 10 to 12 minutes, or until fish flakes easily with a fork. Transfer fish to a serving plate and keep warm.
Increase the heat and add butter to sauce. Simmer until the sauce thickens. Stir in coriander. Serve sauce over fish.