Preheat a char grill or barbecue over high heat. Remove prawn heads and veins, brush prawns with oil and cook for 2-3 minutes each side.
Set aside and keep warm.
Place the fish sauce, caster sugar, water and lime leaves into a small frying pan over high heat, bring to the boil, add peanuts and simmer for 1-2 minutes or until thickened.
Pour over prawns, add coriander and toss to combine.
Serve with lots of fresh lime wedges and extra coriander.