To make the mustard dipping sauce, stir together the mustard, lemon juice, 1/2 teaspoon coriander, 1/2 teaspoon cumin and 1/4 teaspoon pepper in a small bowl. Set aside.
Combine the remaining 1 1/2 teaspoons coriander, 1 1/2 teaspoons cumin and 1/4 teaspoon pepper in a large bowl. Add the prawns, tossing to coat.
Preheat the barbecue. Thread the prawns onto four long skewers. Place the prawns on the barbecue and cook until opaque throughout, about 1 minute per side. Serve at room temperature or chilled, with the mustard dipping sauce.