Whisk together the lime juice, 1/4 teaspoon of the chilli powder, the crushed chilli and a pinch of salt in a large bowl. Add the pineapple, capsicum, cucumber and onion, tossing well to combine. Cover the salsa and refrigerate until serving time.
Whisk together the pineapple juice, oil, oregano and the remaining chilli powder in a large bowl. Add the prawns and toss well. Marinate for 30 minutes.
Preheat the griller or barbecue. Cook the prawns for 2 minutes each side or until just cooked. Serve with the pineapple salsa.