Fill a large saucepan with salted water and bring to the boil. Add the curry powder, let it dissolve, then add the pasta. When the pasta is cooked but still firm, drain and cool with running water. Drain again thoroughly and place in a large salad bowl.
Fill a large pot with salted water and bring to the boil. Boil the prawns until they turn pink and curl slightly, 2-3 minutes. Drain, place under cold running water and drain again. Peel and devein the prawns and set aside.
Add the prawns, smoked salmon and green peas to the pasta.
In a small bowl stir together the tarragon, lemon juice, salt and pepper. Add the oil and stir vigorously until blended.
Pour dressing over pasta dish. Toss well and serve.