In a large bowl, combine prawns, cucumber, tomato, green onion, coriander and chilli. Stir in tomato paste and vinegar. Squeeze lime juice over mixture.
Chop the prawns into smaller pieces and transfer to a bowl. Stir in the avocado and coriander. In a separate bowl, stir together the orange soft drink and ketchup until the foaming stops. Pour over the prawn mixture and toss to coat. It should be soupy. Refrigerate for about 30 minutes to blend the flavours before serving with the crackers.
Serve in glasses with shredded lettuce or slices of celery.